@article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/102942, title ="Catalytic N₂-to-NH₃ (or -N₂H₄) Conversion by Well-Defined Molecular Coordination Complexes", author = "Chalkley, Matthew J. and Drover, Marcus W.", journal = "Chemical Reviews", volume = "120", number = "12", pages = "5582-5636", month = "June", year = "2020", doi = "10.1021/acs.chemrev.9b00638", issn = "0009-2665", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200430-151240025", note = "© 2020 American Chemical Society. \n\nReceived 9 October 2019. Published online 30 April 2020. \n\nThis article is part of the Reactivity of Nitrogen from the Ground to the Atmosphere special issue. \n\nThe authors are grateful to NIH (GM-070757) for ongoing support of their work exploring (L_n)Fe(N_xH_y) model systems of biological nitrogenases and the Department of Energy (DOE-0235032) for supporting their research towards electrocatalytic N₂RR systems. M.W.D. acknowledges NSERC (Banting PDF award to MWD), and M.W.D./M.J.C. thank the Resnick Sustainability Institute at Caltech for fellowships. Dr. Cooper Citek, Dr. Pablo G. Barros, Javier Fajardo Jr., Nina X. Gu, Dr. Heejun Lee, Dr. Alonso Rosas, and Dirk J. Schild are thanked for providing helpful discussion. \n\nAuthor Contributions: M.J.C. and M.W.D. contributed equally. \n\nThe authors declare no competing financial interest.", revision_no = "18", abstract = "Nitrogen fixation, the six-electron/six-proton reduction of N₂, to give NH₃, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N₂ into its bioavailable form, NH₃. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N₂ functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/102447, title ="Relating N-H Bond Strengths to the Overpotential for Catalytic Nitrogen Fixation", author = "Chalkley, Matthew J. and Peters, Jonas C.", journal = "European Journal of Inorganic Chemistry", volume = "2020", number = "15-16", pages = "1353-1357", month = "April", year = "2020", doi = "10.1002/ejic.202000232", issn = "1434-1948", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200409-115738751", note = "© 2020 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nIssue Online: 17 April 2020; Version of Record online: 09 April 2020; Manuscript received: 04 March 2020. \n\nThis research was supported by the National Institutes of Health (GM‐070757). M. J. C. thanks the Resnick Sustainability Institute at Caltech for a graduate research fellowship.", revision_no = "21", abstract = "The reduction of N₂ to NH₃ (N₂RR) is a globally significant reaction that is challenging due to the inertness of N₂. Transition metals can activate N₂ and mediate catalytic N₂RR, but challenges remain with respect to catalytic efficiency in terms of overpotential and selectivity. We discuss the role of the N–H bond dissociation free energy (BDFE) of metal diazenidos (M‐NNH), the first intermediates of N₂RR, in determining N2RR efficiency.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/102349, title ="H₂ Evolution from a Thiolate-Bound Ni(III) Hydride", author = "Gu, Nina X. and Oyala, Paul H.", journal = "Journal of the American Chemical Society", volume = "142", number = "17", pages = "7827-7835", month = "April", year = "2020", doi = "10.1021/jacs.0c00712", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200406-103934930", note = "© 2020 American Chemical Society. \n\nReceived: January 19, 2020; Published: April 4, 2020. \n\nThe authors acknowledge Gerri Roberts for preliminary data and Dr. Michael Takase and Lawrence Henling for assistance with X-ray crystallography. This work was supported by the Department of Energy (DOE-0235032) and NSF-GRFP (N.X.G.). The Caltech EPR facility was supported by the NSF (NSF-1531940) and the Dow Next Generation Educator Fund. The X-ray Crystallography Facility in the Beckman Institute at Caltech was supported by the Dow Next Generation Instrumentation Grant. \n\nThe authors declare no competing financial interest.", revision_no = "21", abstract = "Terminal Ni^(III) hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = 1/2, terminally bound thiolate–Ni^(III)–H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate–Ni^(III)–H species is comparable to that of the hydride ligand proposed for the Ni–C hydrogenase intermediate (Ni^(III)–H–Fe^(II)). Upon warming, the featured thiolate–Ni^(III)–H species undergoes bimolecular reductive elimination of H₂. Associated kinetic studies are discussed and compared with a structurally related Fe^(III)–H species that has also recently been reported to undergo bimolecular H–H coupling.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/101650, title ="Molecular enhancement of heterogeneous CO₂ reduction", author = "Nam, Dae-Hyun and De Luna, Phil", journal = "Nature Materials", volume = "19", number = "3", pages = "266-276", month = "March", year = "2020", doi = "10.1038/s41563-020-0610-2", issn = "1476-1122", url = "https://resolver.caltech.edu/CaltechAUTHORS:20200302-110558568", note = "© 2020 Springer Nature Limited. \n\nReceived 16 January 2019; Accepted 08 January 2020; Published 25 February 2020. \n\nThis work was in part supported financially by the Natural Sciences and Engineering Research Council of Canada, the Ontario Research Fund: Research Excellence Program (ORF-RE-RE08-034), the Natural Resources Canada Clean Growth Program (CGP-17-0455) and CIFAR Bio-Inspired Solar Energy Program (FL-000719). This work was also supported by the Joint Center for Artificial Photosynthesis, a DOE Energy InnovationHub, supported through the Office of Science of the US Department of Energy under award no. DESC0004993, and was also based on work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award no. OSR-2018-CPF-3665-03 and OSR-2019-CCF-1972.04. P.D.L. acknowledges the Natural Sciences and Engineering Research Council of Canada for support in the form of a Canada Graduate Scholarship and A.T. acknowledges the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Action H2020-MSCA-IF-2017 (793471). \n\nThese authors contributed equally: Dae-Hyun Nam, Phil De Luna. \n\nThe authors declare no competing interests.", revision_no = "9", abstract = "The electrocatalytic carbon dioxide reduction reaction (CO₂RR) addresses the need for storage of renewable energy in valuable carbon-based fuels and feedstocks, yet challenges remain in the improvement of electrosynthesis pathways for highly selective hydrocarbon production. To improve catalysis further, it is of increasing interest to lever synergies between heterogeneous and homogeneous approaches. Organic molecules or metal complexes adjacent to heterogeneous active sites provide additional binding interactions that may tune the stability of intermediates, improving catalytic performance by increasing Faradaic efficiency (product selectivity), as well as decreasing overpotential. We offer a forward-looking perspective on molecularly enhanced heterogeneous catalysis for CO₂RR. We discuss four categories of molecularly enhanced strategies: molecular-additive-modified heterogeneous catalysts, immobilized organometallic complex catalysts, reticular catalysts and metal-free polymer catalysts. We introduce present-day challenges in molecular strategies and describe a vision for CO2RR electrocatalysis towards multi-carbon products. These strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of CO₂RR.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/98698, title ="Molecular tuning of CO₂-to-ethylene conversion", author = "Li, Fengwang and Thevenon, Arnaud", journal = "Nature", volume = "577", number = "7791", pages = "509-513", month = "January", year = "2020", doi = "10.1038/s41586-019-1782-2", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190917-154117893", note = "© 2019 Springer Nature Limited. \n\nReceived 21 December 2018; Accepted 01 October 2019; Published 20 November 2019. \n\nThis work was financially supported by the Ontario Research Fund:\nResearch Excellence Program, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the CIFAR Bio-Inspired Solar Energy program, and the Joint Centre of Artificial Synthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. All DFT computations were performed on the IBM BlueGene/Q supercomputer with support from the Southern Ontario Smart Computing Innovation Platform (SOSCIP). SOSCIP is funded by the Federal Economic Development Agency of Southern Ontario, the Province of Ontario, IBM Canada Ltd., Ontario Centres of Excellence, Mitacs and 15 Ontario academic member institutions. This research was enabled in part by support provided by Compute Ontario (www.computeontario.ca) and Compute Canada (www.computecanada.ca). This research used synchrotron resources of the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was\nsupported by the U.S. DOE under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners. We thank T. Wu and L. Ma for technical support at 9BM beamline of APS. D.S. acknowledges the NSERC E.W.R Steacie Memorial Fellowship. A.T. acknowledges Marie Skłodowska-Curie Fellowship H2020-MSCA-IF-2017 (793471). J.L. acknowledges the Banting postdoctoral fellowship. C.M.G. acknowledges NSERC for funding in\nthe form of a postdoctoral fellowship from the government of Canada. J.P.E. thanks NSERC, Hatch and the Government of Ontario for their support through graduate scholarships. \n\nAuthor contributions: E.H.S., T.A. and J.C.P. supervised this project. F.L. and Y.L. carried out electrochemical experiments. A.T. and A.R.H. carried out molecule synthesis and characterizations. Z.W. carried out DFT calculations. C.M.G. and F.L. conducted in situ Raman measurement. F.L. and A.O. carried out the membrane-electrode-assembly experiments. J. L. and F.L. performed X-ray spectroscopy measurements. Y.W. carried out SEM and EIS measurements. J.P.E. measured the contact angle. C.M. carried out the Comsol modelling. L.T. carried out EPR measurement under the supervision of R.D.B.. M.L. performed part of electrochemical experiments. Z. Q. L., X.W. and H.L. provided help in NMR analysis. C.M.G., C.P.O. and Y.X. provided help in membrane-electrode-assembly measurements. C.S.T. carried out AFM measurement. D.H.N. conducted XRD measurement. R.Q.B. carried out XPS measurement. C.T.D., T.Z, Y.C.L. and Z.H. provided help in materials synthesis and characterizations. F.L. and E.H.S. wrote the manuscript. All authors discussed the results and assisted during manuscript preparation. \n\nThe authors declare no competing interests.", revision_no = "44", abstract = "The electrocatalytic carbon dioxide (CO₂) reduction reaction (CO₂RR) to value-added fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of intermittent renewable electricity. The highly selective generation of economically desirable C₂ products such as ethylene from CO₂RR remains a challenge. Tuning the stabilities of intermediates to favour a desired reaction pathway offers the opportunity to enhance selectivity, and this has recently been explored on copper (Cu) via control over morphology, grain boundaries7, facets, oxidation state and dopants. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 mA cm⁻² in the best catalyst reported so far), resulting in a low energy efficiency. Here we present a molecular tuning strategy—the functionalization of the surface of electrocatalysts with organic molecules—that stabilizes intermediates for enhanced CO₂RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived via electro-dimerization of arylpyridiniums, on Cu. We find that the adhered molecules improve the stabilization of an atop-bound CO intermediate, thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO₂RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 mA cm⁻² in a liquid-electrolyte flow cell in neutral medium. We report stable ethylene electrosynthesis for 190 hours in a membrane-electrode-assembly-based system that provides a full-cell energy efficiency of 20 per cent. These findings indicate how molecular strategies can complement heterogeneous catalysts by stabilizing intermediates via local molecular tuning.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/98794, title ="In-situ Nanostructuring and Stabilization of Polycrystalline Copper by an Organic Salt Additive Promotes Electrocatalytic CO₂ Reduction to Ethylene", author = "Thevenon, Arnaud and Rosas-Hernández, Alonso", journal = "Angewandte Chemie International Edition", volume = "58", number = "47", pages = "16952-16958", month = "November", year = "2019", doi = "10.1002/anie.201907935", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190923-102748967", note = "© 2019 Wiley-‐VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nAccepted manuscript online: 19 September 2019; Manuscript accepted: 19 September 2019; Manuscript revised: 13 August 2019; Manuscript received: 25 June 2019. \n\nNMR, AFM and XPS, SEM and EDX measurements were collected at the NMR Facility (Division of CCE), the Molecular Materials Research Center (Beckman Institute) and the Analytic Facilities (Division of Geological and Planetary Sciences) of the California Institute of Technology, respectively. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993 and Marie Curie Fellowship H2020-MSCA-IF-2017 (793471) (A.T.). J.C.P. acknowledges additional support from the Resnick Sustainability Institute at Caltech. \n\nThe authors declare no conflict of interest.", revision_no = "22", abstract = "Bridging homogeneous molecular systems with heterogeneous catalysts is a promising approach for the development of new electrodes, combining the advantages of both approaches. In the context of CO₂ electroreduction, molecular enhancement of planar copper electrodes has enabled promising advancement towards high Faradaic efficiencies for multicarbon products. Besides, nanostructured copper electrodes have also demonstrated enhanced performance at comparatively low overpotentials. Herein, we report a novel and convenient method for nanostructuring copper electrodes using N,N’‐ethylene‐phenanthrolinium dibromide as molecular additive. Selectivities up to 70% for C≥₂ products are observed for more than 40 h without significant change in the surface morphology. Mechanistic studies reveal several roles for the organic additive, including: the formation of cube‐like nanostructures by corrosion of the copper surface, the stabilization of these nanostructures during electrocatalysis by formation of a protective organic layer, and the promotion of C≥₂ products.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/99248, title ="Electrocatalytic Ammonia Oxidation Mediated by a Polypyridyl Iron Catalyst", author = "Zott, Michael D. and Garrido-Barros, Pablo", journal = "ACS Catalysis", volume = "9", number = "11", pages = "10101-10108", month = "November", year = "2019", doi = "10.1021/acscatal.9b03499", issn = "2155-5435", url = "https://resolver.caltech.edu/CaltechAUTHORS:20191014-102537841", note = "© 2019 American Chemical Society. \n\nReceived: August 17, 2019; Revised: September 16, 2019; Published: September 19, 2019. \n\nThis work was supported by Caltech, the Gordon and Betty Moore Foundation, and in part the Department of Energy, Office of Basic Energy Science, Catalysis Program (DOE-0235032) for general materials and supplies. We are grateful for access to the X-ray Facility and the Molecular Materials Research Center within the Beckman Institute and acknowledge Dr. Michael Takase and Larry Henling for assistance with XRD analysis. The authors thank Matthew J. Chalkley for helpful discussions. \n\nAuthor Contributions: M.D.Z. and P.G.-B.: These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "Electrocatalytic ammonia oxidation (AO) mediated by iron(II) tris(2-pyridylmethyl)amine (TPA) bis-ammine triflate, [(TPA)Fe(NH₃)₂]OTf₂, is reported. Interest in (electro)catalytic AO is growing rapidly, and this report adds a first-row transition metal (iron) complex to the known Ru catalysts recently reported. The featured system is well behaved and has been studied in detail by electrochemical methods. Cyclic voltammetry experiments in the presence of ammonia indicate an onset potential corresponding to ammonia oxidation at 0.7 V vs Fc/Fc⁺. Controlled potential coulometry (CPC) at an applied bias of 1.1 V confirms the generation of 16 equiv of N₂ with a Faradaic efficiency for N₂ of ∼80%. Employing ¹⁵NH₃ yields exclusively ³⁰N₂, demonstrating the conversion of ammonia to N₂. A suite of electrochemical studies is consistent with an initial EC step that generates an Fe^(III)–NH₂ intermediate (at 0.4 V) followed by an anodically shifted catalytic wave. The data indicate a rate-determining step that is first order in both [Fe] and [NH₃] and point to a fast catalytic rate (k_(obs)) of ∼10⁷ M⁻¹·s⁻¹ as computed by foot of the wave analysis (FOWA).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/100162, title ="Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer", author = "Drover, Marcus W. and Schild, Dirk J.", journal = "Angewandte Chemie International Edition", volume = "58", number = "43", pages = "15504-15511", month = "October", year = "2019", doi = "10.1002/anie.201909050", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20191203-101325731", note = "© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nIssue Online: 14 October 2019; Version of Record online: 18 September 2019; Accepted manuscript online: 29 August 2019; Manuscript received: 22 July 2019. \n\nThe authors are grateful to the Department of Energy for support via Grant No. DOE‐0235032. The Caltech EPR facility was supported by the National Science Foundation via grant No. NSF MRI‐153194, as well as the Dow Next Generation Educator Fund. The Beckman Institute is thanked for X‐ray support. M.W.D. acknowledges NSERC (Banting PDF award), and M.W.D./D.J.S thank the Resnick Sustainability Institute at Caltech for fellowships. \n\nThe authors declare no conflict of interest.", revision_no = "16", abstract = "We report the characterization of an S=1/2 iron π‐complex, [Fe(η⁶‐IndH)(depe)]⁺ (Ind=Indenide (C₉H₇⁻), depe=1,2‐bis(diethylphosphino)ethane), which results via C−H elimination from a transient Fe^(III) hydride, [Fe(η³:η²‐Ind)(depe)H]⁺. Owing to weak M−H/C−H bonds, these species appear to undergo proton‐coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open‐shell π‐arene complex to have a BDFE_(C‐H) value of ≈50 kcal\u2009mol⁻¹, roughly equal to the BDFE_(Fe‐H) of its Fe^(III)−H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵‐Cp/Cp*) compounds, for which terminal Fe^(III)−H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring‐ versus a metal‐bound H‐atom by way of continuous‐wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/98338, title ="Snapshots of a Migrating H-atom: Characterization of a Reactive Fe(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Fe(I) Isomer", author = "Drover, Marcus W. and Schild, Dirk J.", journal = "Angewandte Chemie International Edition", volume = "58", number = "43", pages = "15504-15511", month = "October", year = "2019", doi = "10.1002/anie.201909050", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190829-111225922", note = "© 2019 WILEY‐VCH. \n\nManuscript received: July 22, 2019; Accepted manuscript online: August 29, 2019; Version of record online: September 18, 2019. \n\nThe authors are grateful to the Department of Energy for support via Grant No. DOE-0235032. The Caltech EPR facility was supported by the National Science Foundation via grant No. NSF MRI-153194, as well as the Dow Next Generation Educator Fund. The Beckman Institute is thanked for X-ray support. M.W.D. acknowledges NSERC (Banting PDF award to MWD), and M.W.D./D.J.S. thank the Resnick Sustainability Institute at Caltech for fellowships. \n\nThe authors declare no competing financial interest.", revision_no = "36", abstract = "We report the characterization of an S = ½ iron π‐complex, [Fe(η^6‐IndH)(depe)]^+ (Ind = Indenide (C_9H_(7^‐_), depe = 1,2‐bis(diethylphosphino)ethane), which results via C‐H elimination from a transient Fe^(III) hydride, [Fe(η^3:η^2‐Ind)(depe)H]^+. Owing to weak M‐H/C‐H bonds, these species undergo proton‐coupled electron transfer (PCET) to release H_2 through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open‐shell π‐arene complex to have a BDFE_(C‐H) value of ≈ 50 kcal mol^(‐1), roughly equal to the BDFE_(Fe‐H) of its Fe^(III)‐H precursor (ΔG^o ≈ 0 between them). Markedly, this reactivity differs from related Fe(η^5‐Cp/Cp^*) compounds, for which terminal Fe^(III)‐H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated (indene) ring. Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and out‐lines a valuable approach for the differentiation of a ring‐ versus a metal‐ bound H‐atom by way of continuous‐wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/98044, title ="Mononuclear Fe(I) and Fe(II) Acetylene Adducts and their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes", author = "Citek, Cooper and Oyala, Paul H.", journal = "Journal of the American Chemical Society", volume = "141", number = "38", pages = "15211-15221", month = "September", year = "2019", doi = "10.1021/jacs.9b06987", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190820-134220046", note = "© 2019 American Chemical Society. \n\nReceived: July 2, 2019; Published: August 20, 2019. \n\nThis work was supported by the National Institutes of Health (General Medical Sciences, grant GM070757). C.C. acknowledges the NIH Ruth L. Kirschstein National Service Fellowship for financial support. Additional support has been provided by the Caltech EPR Facility, supported via NSF-1531940, and the Dow Next Generation Educator Fund. We also acknowledge the Beckman Institute for use of the its X-ray facility and thank Larry M. Henling and Dr. David G. VanderVelde for technical assistance with X-ray and NMR experiments, respectively. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. “Fischer–Tropsch” type coupling of non-native C1 substrates to higher-order C_(≥2) products is also known for nitrogenase, though potential metal–carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP_3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 Fe^I(η^2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP_3)Fe≡CCH_3 and the paramagnetic cation S = 1/2 [(SiP_3)Fe≡CCH_3]^+. Both alkylcarbyne compounds possess short Fe–C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH_3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH_3 unit. The electronic structures of (SiP_3)Fe≡CCH_3 and [(SiP_3)Fe≡CCH_3]^+ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP_3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/97353, title ="O-Functionalization of a cobalt carbonyl generates a terminal cobalt carbyne", author = "Deegan, Meaghan M. and Peters, Jonas C.", journal = "Chemical Communications", volume = "55", number = "64", pages = "9531-9534", month = "August", year = "2019", doi = "10.1039/c9cc04032c", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190723-104318121", note = "© 2019 The Royal Society of Chemistry. \n\nThe article was received on 24 May 2019, accepted on 30 Jun 2019 and first published on 01 Jul 2019. \n\nThis research was supported by the Department of Energy (DOE-0235032) with facilities support provided by the DOW Next Generation Educator Fund and the Beckman Institute at Caltech. We thank Larry Henling and Mike Takase for crystallographic assistance. We additionally thank Nina Gu and Kareem Hannoun for assistance with computational studies and helpful suggestions. \n\nThere are no conflicts to declare.", revision_no = "22", abstract = "Despite efforts toward extending multiple bonding motifs to late metal systems, examples of late transition metal carbynes remain scarce. Herein, we describe the synthesis of a series of L_3Co(CO) complexes supported by a trisphosphine ligand framework, with the most reduced of these complexes being amenable to O-functionalization. This transformation provides access to the second reported example of a terminal Co-carbyne complex, in this case stabilized in a pseudotetrahedral geometry (i.e., L_3Co ≡ C-OSiR_3). Its geometry makes its electronic structure suitable for comparison to structurally-related examples of terminal Co-imido and oxo species.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/95208, title ="Characterization of the earliest intermediate of Fe-N_2 protonation: CW and Pulse EPR detection of an Fe-NNH species and its evolution to Fe-NNH_2^+", author = "Nesbit, Mark A. and Oyala, Paul H.", journal = "Journal of the American Chemical Society", volume = "141", number = "20", pages = "8116-8127", month = "May", year = "2019", doi = "10.1021/jacs.8b12082", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190503-131410286", note = "© 2019 American Chemical Society. \n\nReceived: November 9, 2018; Published: May 2, 2019. \n\nThis work was supported by the NIH (R01-070757). The EPR facility at the California Institute of Technology has been supported by the NSF via its MRI program (NSF-1531940) and the DOW Next Generation Educator Fund. We thank Dirk Schild for performing a BDFE_(N–H) calculation on trans-(H)(DMeOPrPE)_2Fe(N═NH) and Dr. Jonathan Rittle for contributions to the ligand synthesis. \n\nThe authors declare no competing financial interest.", revision_no = "25", abstract = "Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N_2-to-NH_3 conversion (N_2RR) mediated by synthetic iron complexes and relatedly as intermediates of N_2RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR_3), and also of further downstream intermediates (e.g., Fe(NNH_2)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N_2– species, supported by a bulky trisphosphinoborane ligand (^(Ar)P_3^B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The ^1H-hyperfine for ^(Ar)P_3^BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom (a_(iso) = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH_2)]+ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH_2)]+ intermediate of N2RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N_2-to-NH_3 conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N2RR in a synthetic system.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94741, title ="Catalytic hydrazine disproportionation mediated by a thiolate-bridged VFe complex", author = "Gu, Nina X. and Ung, Gaël", journal = "Chemical Communications", volume = "55", number = "37", pages = "5363-5366", month = "May", year = "2019", doi = "10.1039/c9cc00345b", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190417-100521011", note = "© 2019 The Royal Society of Chemistry. \n\nThe article was received on 15 Jan 2019, accepted on 06 Apr 2019 and first published on 09 Apr 2019. \n\nThis work was supported by the Department of Energy (DOE-0235032) and the NSF-GRFP (N. X. G.), and the authors acknowledge the Dow Next Generation Educator Fund. We thank Dr. Michael Takase and Lawrence Henling for assistance with X-ray crystallography and Nathanael Hirscher for assistance with Toepler pump experiments. \n\nThere are no conflicts to declare.", revision_no = "19", abstract = "A heterobimetallic VFe complex is demonstrated to catalyse hydrazine disproportionation with yields of up to 1073 equivalents of NH_3 per catalyst, comparable to the highest turnover known for any molecular catalyst. Notably, the heterobimetallic complex is appreciably more active than monometallic analogues of the V and Fe sites, suggesting that bimetallic cooperativity may facilitate the observed catalysis.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92936, title ="Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center, Three-Electron Polar σ Bonds", author = "Nance, Patricia J. and Thompson, Niklas B.", journal = "Angewandte Chemie International Edition", volume = "58", number = "19", pages = "6220-6224", month = "May", year = "2019", doi = "10.1002/anie.201814206", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190214-111511215", note = "© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nVersion of Record online: 01 April 2019; Accepted manuscript online: 13 February 2019; Manuscript revised: 26 January 2019; Manuscript received: 13 December 2018. \n\nWe are grateful for funding from the Department of Energy (DOE‐0235032), from the NSF through its MRI program (NSF‐1531940) in support of the Caltech EPR facility, and to the DOW Next Generation Educator Fund. PJN is grateful for an NSF GRF. We also thank Dr. Michael Takase and Larry Henling for assistance with X‐ray crystallography. The authors declare no conflict of interest.", revision_no = "19", abstract = "Species with 2‐center, 3‐electron (2c/3e−) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d^9) trigonal pyramidal Rh and Ir complexes that feature 2c/3e− σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e− bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z^2 parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e− σ “half‐bonds”.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/94908, title ="Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H_2 Elimination, HER, and NH_3 Generation", author = "Schild, Dirk J. and Peters, Jonas C.", journal = "ACS Catalysis", volume = "9", number = "5", pages = "4286-4295", month = "May", year = "2019", doi = "10.1021/acscatal.9b00523", issn = "2155-5435", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190423-160445593", note = "© 2019 American Chemical Society. \n\nReceived: February 4, 2019; Revised: March 21, 2019; Published: March 26, 2019. \n\nDr. Michael Takase, Larry Henling, and Dr. Marcus Drover are acknowledged for their assistance with crystallographic studies. Dr. Niklas Thompson is thanked for assisting with fitting the variable temperature NMR data. We are grateful to the NIH (GM-075757) for support of this research, and also to the National Science Foundation for support of the Caltech EPR Facility via the NSF-MRI Grant NSF-153194, and to the Dow Next Generation Educator Fund. D.J.S. acknowledges the support of the Resnick Sustainability Institute at Caltech for a Graduate Fellowship. \n\nThe authors declare no competing financial interest.", revision_no = "22", abstract = "Despite their proposed accumulation at the Fe sites of the FeMo-cofactor of MoFe-nitrogenase, the presence of hydride ligands in molecular model systems capable of the nitrogen reduction reaction (N_2RR) appears to diminish catalytic N_2-to-NH_3 conversion. We find that, for an iron-based system bearing the trisphosphine ligand P_2P^(Ph), a dramatic difference in yields is observed for N2RR catalyzed by precatalysts with zero, one, or two hydride ligands; however, irradiating the three different catalysts with a mercury lamp results in similar NH3 yields. Although the efficacy for N2RR versus the hydrogen evolution reaction (HER) is modest for this system by comparison to certain iron (and other metal) catalysts, the system provides an opportunity to study the role of hydrides in the selectivity for N_2RR versus HER, which is a central issue in catalyst design. Stoichiometric reactions with hydride containing precatalysts reveal a hydrogen evolution cycle in which no nitrogen fixation occurs. Irradiation of the dihydride precatalysts, observed during turnover, results in H2 elimination and formation of (P_2P^(Ph))Fe(N_2)_2, which itself is unreactive with acids at low temperature. N_2 functionalization does occur with acids and silyl electrophiles for the reduced species [(P_2P^(Ph))Fe(N_2)]^− and [(P_2P^(Ph))Fe(N_2)]^(2–), which have been characterized independently. The requirement of accessing such low formal oxidation states explains the need for strong reductants. The low selectivity of the system for functionalization at Nβ versus Fe creates off-path hydride species that participate in unproductive HER, helping to explain the low selectivity for N_2RR over HER. The data presented here thus lend further insight into the growing understanding of the selectivity, activity, and reductant strengths relevant to iron (and other) N_2RR catalysts.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/93165, title ="Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation", author = "Chalkley, Matthew J. and Oyala, Paul H.", journal = "Journal of the American Chemical Society", volume = "141", number = "11", pages = "4721-4729", month = "March", year = "2019", doi = "10.1021/jacs.9b00193", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190221-125557747", note = "© 2019 American Chemical Society. \n\nReceived: January 7, 2019; Published: February 21, 2019. \n\nThis work was supported by Department of Energy (DOE-0235032). MJC acknowledges support from the Center for Environmental Microbial Interactions (CEMI) and the Resnick Sustainability Institute at Caltech. The Caltech EPR facility was supported by the National Science Foundation (NSF MRI-153194) and the Dow Next Generation Educator Fund. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "Metallocenes, including their permethylated variants, are extremely important in organometallic chemistry. In particular, many are synthetically useful either as oxidants (e.g., Cp_2Fe^+) or as reductants (e.g., Cp_2Co, Cp*_2Co, and Cp*_2Cr). The latter have proven to be useful reagents in the reductive protonation of small-molecule substrates, including N_2. As such, understanding the behavior of these metallocenes in the presence of acids is paramount. In the present study, we undertake the rigorous characterization of the protonation products of Cp*_2Co using pulse electron paramagnetic resonance (EPR) techniques at low temperature. We provide unequivocal evidence for the formation of the ring-protonated isomers Cp*(exo/endo-η^4-C_5Me_5H)Co^+. Variable temperature Q-band (34 GHz) pulse EPR spectroscopy, in conjunction with density functional theory (DFT) predictions, are key to reliably assigning the Cp*(exo/endo-η^4-C_5Me_5H)Co^+ species. We also demonstrate that exo-protonation selectivity can be favored by using a bulkier acid and suggest this species is thus likely a relevant intermediate during catalytic nitrogen fixation given the bulky anilinium acids employed. Of further interest, we provide physical data to experimentally assess the C–H bond dissociation free energy (BDFE_(C–H)) for Cp*(exo-η^4-C_5Me_5H)Co^+. These experimental data support our prior DFT predictions of an exceptionally weak C–H bond (<29 kcal mol^(–1)), making this system among the most reactive (with respect to C–H bond strength) to be thoroughly characterized. These data also point to the propensity of Cp*(exo-η^4-C_5Me_5H)Co to mediate hydride (H–) transfer. Our findings are not limited to the present protonated metallocene system. Accordingly, we outline an approach to rationalizing the reactivity of arene-protonated metal species, using decamethylnickelocene as an additional example.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/92933, title ="Electronic Structures of an [Fe(NNR_2)]^(+/0/–) Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation", author = "Thompson, Niklas B. and Oyala, Paul H.", journal = "Inorganic Chemistry", volume = "58", number = "5", pages = "3535-3549", month = "March", year = "2019", doi = "10.1021/acs.inorgchem.9b00133", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20190214-104228298", note = "© 2019 American Chemical Society. \n\nReceived: January 14, 2019; Published: February 14, 2019. \n\nThis work was supported by the Resnick Sustainability Institute at Caltech (a graduate fellowship to N.B.T.), as well as the NIH (Grant GM 070757). The EPR facility at the California Institute of Technology is supported by the NSF via its MRI program (Grant NSF-1531940) and the Dow Next Generation Educator Fund. \n\nThe authors declare no competing financial interest.", revision_no = "27", abstract = "The intermediacy of metal–NNH_2 complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N_2) fixation. In this context, we have shown that triphosphine-supported Fe(N_2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH_2) complexes over an array of charge and oxidation states. Upon exposure to further H^+/e^– equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N–H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe_2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe_2” moiety. Specifically, although two closed-shell configurations of the “NNR_2” ligand have been commonly considered in the literature—isodiazene and hydrazido(2−)—we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR_2]^•–ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N_2 fixation activity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/90547, title ="Visible-Light-Induced, Copper-Catalyzed Three-Component Coupling of Alkyl Halides, Olefins, and Trifluoromethylthiolate to Generate Trifluoromethyl Thioethers", author = "He, Jian and Chen, Caiyou", journal = "ACS Catalysis", volume = "8", number = "12", pages = "11741-11748", month = "December", year = "2018", doi = "10.1021/acscatal.8b04094", issn = "2155-5435", url = "https://resolver.caltech.edu/CaltechAUTHORS:20181031-131746669", note = "© 2018 American Chemical Society. \n\nReceived: October 10, 2018; Revised: October 24, 2018; Published: October 31, 2018. \n\nSupport has been provided by the National Institutes of Health (National Institute of General Medical Sciences, Grant R01-109194). We are also grateful to the Dow Next Generation Educator Fund. We thank Dr. Tanvi S. Ratani, Dr. Paul H. Oyala (Caltech EPR Facility, supported by NSF-1531940), Larry M. Henling, Dr. Jun Myun Ahn, and Theodore M. Donnell for experimental assistance and for helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "26", abstract = "Photoinduced, copper-catalyzed coupling reactions are emerging as a powerful method for generating Csp^3–Y (Y = C or heteroatom) bonds from alkyl electrophiles and nucleophiles. Corresponding three-component couplings of alkyl electrophiles, olefins, and nucleophiles have the potential to generate an additional Csp3–Y bond and to efficiently add functional groups to both carbons of an olefin, which serves as a readily available linchpin. In this report, we establish that a variety of electrophiles and a trifluoromethylthiolate nucleophile can add across an array of olefins (including styrenes and electron-poor olefins) in the presence of CuI/binap and blue-LED irradiation, thereby generating trifluoromethyl thioethers in good yield. The process tolerates a wide range of functional groups, and an initial survey of other nucleophiles (i.e., bromide, cyanide, and azide) suggests that this three-component coupling strategy is versatile. Mechanistic studies are consistent with a photoexcited Cu(I)/binap/SCF_3 complex serving as a reductant to generate an alkyl radical from the electrophile, which likely reacts in turn with the olefin and a Cu(II)/SCF_3 complex to afford the coupling product.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/89675, title ="ENDOR Characterization of (N_2)Fe^(II)(μ-H)_2Fe^I(N_2)^−: A Spectroscopic Model for N_2 Binding by the Di-μ-hydrido Nitrogenase Janus Intermediate", author = "Yang, Hao and Rittle, Jonathan", journal = "Inorganic Chemistry", volume = "57", number = "19", pages = "12323-12330", month = "October", year = "2018", doi = "10.1021/acs.inorgchem.8b02021", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180917-125103511", note = "© 2018 American Chemical Society. \n\nReceived: July 20, 2018; Published: September 17, 2018. \n\nThis work was supported by the NIH (GM 075757 to J.C.P) and the NSF (MCB 1515981 to B.M.H). We thank Dr. Peter Doan for insightful ENDOR discussions. \n\nThe authors declare no competing financial interest.", revision_no = "27", abstract = "The biomimetic diiron complex 4-(N_2)_2, featuring two terminally bound Fe–N_2 centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N_2. One is the Janus intermediate E4(4H), which has accumulated 4[e–/H+], stored as two [Fe–H–Fe] bridging hydrides, and is activated to bind and reduce N_2 through reductive elimination (RE) of the hydride ligands as H_2. The second is a possible RE intermediate. ^^1H and ^(14)N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N_2)_2 complex exhibits a fully delocalized, Robin–Day type-III mixed valency. The two bridging hydrides exhibit a fully rhombic dipolar tensor form, T ≈ [−t, +t, 0]. The rhombic form is reproduced by a simple point-dipole model for dipolar interactions between a bridging hydride and its “anchor” Fe ions, confirming validity of this model and demonstrating that observation of a rhombic form is a convenient diagnostic signature for the identification of such core structures in biological centers such as nitrogenase. Furthermore, interpretation of the ^1H measurements with the anchor model maps the g tensor onto the molecular frame, an important function of these equations for application to nitrogenase. Analysis of the hyperfine and quadrupole coupling to the bound ^(14)N of N_2 provides a reference for nitrogen-bound nitrogenase intermediates and is of chemical significance, as it gives a quantitative estimate of the amount of charge transferred between Fe and coordinated N, a key element in N_2 activation for reduction.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/87573, title ="Electrophile-promoted Fe-to-N_2 hydride migration in highly reduced Fe(N_2)(H) complexes", author = "Deegan, Meaghan M. and Peters, Jonas C.", journal = "Chemical Science", volume = "9", number = "29", pages = "6264-6270", month = "August", year = "2018", doi = "10.1039/C8SC02380H", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180705-154047668", note = "© 2018 the Author(s). This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. All publication charges for this article have been paid for by the Royal Society of Chemistry. \n\nReceived 31st May 2018, Accepted 26th June 2018. First published on 29th June 2018. \n\nThis work was supported by the NIH (GM 070757). The authors are grateful to Larry Henling and Mike Takase for crystallographic assistance. We thank Trixia Buscagan for preliminary data on complex 4. \n\nConflicts of interest: There are no conflicts to declare.", revision_no = "23", abstract = "One of the emerging challenges associated with developing robust synthetic nitrogen fixation catalysts is the competitive formation of hydride species that can play a role in catalyst deactivation or lead to undesired hydrogen evolution reactivity (HER). It is hence desirable to devise synthetic systems where metal hydrides can migrate directly to coordinated N2 in reductive N–H bond-forming steps, thereby enabling productive incorporation into desired reduced N_2-products. Relevant examples of this type of reactivity in synthetic model systems are limited. In this manuscript we describe the migration of an iron hydride (Fe-H) to N_α of a disilylhydrazido(2-) ligand (Fe=NNR_2) derived from N_2 via double-silylation in a preceding step. This is an uncommon reactivity pattern in general; well-characterized examples of hydride/alkyl migrations to metal heteroatom bonds (e.g., (R)M=NR′ → M–N(R)R′) are very rare. Mechanistic data establish the Fe-to-N_α hydride migration to be intramolecular. The resulting disilylhydrazido(1-) intermediate can be isolated by trapping with CN^tBu, and the disilylhydrazine product can then be liberated upon treatment with an additional acid equivalent, demonstrating the net incorporation of an Fe-H equivalent into an N-fixed product.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/87437, title ="Fusing triphenylphosphine with tetraphenylborate: introducing the 9-phosphatriptycene-10-phenylborate (PTB) anion", author = "Drover, Marcus W. and Nagata, Koichi", journal = "Chemical Communications", volume = "54", number = "57", pages = "7916-7919", month = "July", year = "2018", doi = "10.1039/c8cc04321c", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180628-102927648", note = "© 2018 The Royal Society of Chemistry. \n\nReceived 30th May 2018, Accepted 22nd June 2018. First published on 22nd June 2018. \n\nThis work was supported by the NIH (GM070757), NSERC (Banting PDF award to MWD), and the Resnick Sustainability Institute at Caltech (Postdoctoral award to MWD). KN thanks the Japan Society for the Promotion of Science (JSPS) for an overseas postdoctoral fellowship. We thank Larry Henling and Dr Mike Takase for assistance with X-ray crystallography.", revision_no = "25", abstract = "In a fusion of two ubiquitous organometallic reagents, triphenylphosphine (PPh_3) and tetraphenylborate (BPh_4−), the 9-phosphatriptycene-10-phenylborate (PTB) anion has been prepared for the first time. This borato species has been fully characterized by a suite of spectroscopic methods, and initial reactivity studies introduce it as a competent ligand for transition metals, including Co(II) and Fe(II).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/86017, title ="An S = ½ iron complex featuring N₂, thiolate, and hydride ligands: Reductive elimination of H₂ and relevant thermochemical Fe-H parameters", author = "Gu, Nina X. and Oyala, Paul H.", journal = "Journal of the American Chemical Society", volume = "140", number = "20", pages = "6374-6382", month = "May", year = "2018", doi = "10.1021/jacs.8b02603", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180424-082955413", note = "© 2018 American Chemical Society. \n\nReceived: March 7, 2018; Published: April 23, 2018. \n\nThe authors are grateful to the NIH for support via Grant No. GM-070757 and to the National Science Foundation for support of the Caltech EPR Facility via Grant No. NSF-153194. N.X.G. acknowledges an NSF Graduate Research Fellowship. Dr. Gaël Ung provided technical assistance toward the synthesis of 2 and 3. Dr. Michael Takase and Larry Henling are thanked for providing crystallographic assistance. We thank Dr. Jay Winkler for helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "28", abstract = "Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N₂ binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated Fe^(III)(H)(N₂) complex, which releases H₂ upon warming to yield an Fe^(II)–N₂–Fe^(II) complex. Bimolecular reductive elimination of H₂ from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe–H bond with a computationally determined BDFE of 55.6 kcal/mol.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/85963, title ="Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis", author = "Chalkley, Matthew J. and Del Castillo, Trevor J.", journal = "Journal of the American Chemical Society", volume = "140", number = "19", pages = "6122-6129", month = "May", year = "2018", doi = "10.1021/jacs.8b02335", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180418-164723702", note = "© 2018 American Chemical Society. \n\nReceived: February 28, 2018\nPublished: April 18, 2018\nPublication Date (Web): April 18, 2018.\n\nThis work was supported by the NIH (GM-075757) and the\nResnick Sustainability Institute at Caltech. MJC, TJDC, and\nBDM are grateful for NSF Graduate Research Fellowships and\nMJC acknowledges a Caltech Environment Microbial Interactions\n(CEMI) Fellowship. This work made use of the Extreme\nScience and Engineering Discovery Environment (XSEDE),\nwhich is supported by the NSF Grant ACI-1053575. We also\nthank Pakpoom Buabthong for technical assistance with XPS\nmeasurements.", revision_no = "25", abstract = "Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N_2, CO_2, and O_2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P_3^BFe^+, for catalyzing the nitrogen reduction reaction (N_2RR, N_2-to-NH_3 conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pKa and N_2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N_2 reduction (e.g., Fe(NNH) or Fe(NNH_2)) in the presence of the metallocene reductant Cp*_2Co. This suggests that the interaction of acid and reductant is playing a critical role in N–H bond forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N–H bonds critical for N_2RR. Furthermore, DFT studies also suggest that the observed pK_a effect on N_2RR efficiency is attributable to the rate and thermodynamics, of Cp*_2Co protonation by the different anilinium acids. Inclusion of Cp*_2Co^+ as a co-catalyst in controlled potential electrolysis experiments leads to improved yields of NH_3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv NH_3 per Fe at −2.1 V vs Fc^(+/0)).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84844, title ="Expanding the allyl analogy: accessing η^3-P,B,P diphosphinoborane complexes of group 10", author = "Drover, Marcus W. and Peters, Jonas C.", journal = "Dalton Transactions", volume = "47", number = "11", pages = "3733-3738", month = "March", year = "2018", doi = "10.1039/c8dt00058a", issn = "1477-9226", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180215-085923720", note = "© 2018 The Royal Society of Chemistry. \n\nReceived 6th January 2018, Accepted 2nd February 2018. First published on 5th February 2018. \n\nThis work was supported by the NIH (GM070757), NSERC (Banting PDF award to MWD), and the Resnick Sustainability Institute at Caltech (Postdoctoral award to MWD). We thank Larry Henling and Mike Takase for assistance with X-ray crystallography. \n\nThere are no conflicts to declare.", revision_no = "28", abstract = "Using the diphosphinoborane, (PPh_2)_2BMes (Mes = 2,4,6-Me_3C_6H_3), we report the first examples of η^3-P,B,P-ligated complexes using Ni(0) and Pt(II). Reaction of (PPh_2)_2BMes with Ni(COD)_2 or Pt(COD)Me_2 (COD = 1,5-cyclooctadiene) results in gradual COD displacement to give [η^3-P,B,P-(PPh_2)_2BMes]Ni(COD) (3) or [η^3-P,B,P-(PPh_2)_2BMes]Pt(CH_3)_2 (6). Complex 3 serves as a versatile Ni-containing synthon for the preparation of square planar or tetrahedral Ni(0) complexes. Notably, the M–B interaction in these systems is non-negligible – with coordination resulting in an upfield shift of ca. 80 ppm in the ^(11)B NMR spectrum. We also show that treatment of the Pt^(IV) halide precursor, [PtMe_3I]_4 with this ligand framework results in migration of X-type ligands (CH_3− and I−) to boron and reductive elimination of ethane (C_2H_6) to give a distorted square planar zwitterionic Pt^(II) complex, Pt[κ^2-P,P-(PPh_2)_2B(Mes)(CH_3)][κ^2-P,P-(PPh_2)_2B(Mes)(I)] (10). This reactivity suggests the feasibility of (PPh_2)_2BMes-ligand-induced labilization of M–X ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/84084, title ="Fe-mediated HER vs N_2RR: Exploring Factors that Contribute to Selectivity in P_3^EFe(N_2) (E = B, Si, C) Catalyst Model Systems", author = "Matson, Benjamin D. and Peters, Jonas C.", journal = "ACS Catalysis", volume = "8", number = "2", pages = "1448-1455", month = "February", year = "2018", doi = "10.1021/acscatal.7b03068", issn = "2155-5435", url = "https://resolver.caltech.edu/CaltechAUTHORS:20180104-140230786", note = "© 2018 American Chemical Society. \n\nReceived: September 7, 2017; Revised: January 3, 2018; Published: January 3, 2018. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. B.D.M. acknowledges the support of the NSF for a Graduate Fellowship (GRFP). We thank Matthew Chalkley for insightful input. \n\nNo competing financial interests have been declared.", revision_no = "31", abstract = "Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis. This is especially true of catalytic nitrogen (N_2) and carbon dioxide (CO_2) reduction reactions (N_2RR and CO_2RR, respectively) using H^+/e• currency. Here we explore, via DFT calculations, three iron model systems, P_3^EFe (E = B, Si, C), known to mediate both N_2RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is suggested that the respective efficiencies of these systems for N_2RR trend with the predicted N–H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P_3^EFe(NNH_2)^+ and P_3^EFe(NNH_2). Further, a mechanism is presented for undesired HER consistent with DFT studies, and previously reported experimental data, for these systems; bimolecular proton-coupled-electron-transfer (PCET) from intermediates with weak N–H bonds is posited as an important source of H_2, instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways. Wiberg bond indices provide additional insight into key factors related to the degree of stabilization of P_3^EFe(NNH_2) species, factors that trend with overall product selectivity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83538, title ="Copper-Catalyzed Alkylation of Aliphatic Amines Induced by Visible Light", author = "Matier, Carson D. and Schwaben, Jonas", journal = "Journal of the American Chemical Society", volume = "139", number = "49", pages = "17707-17710", month = "December", year = "2017", doi = "10.1021/jacs.7b09582", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171128-135738544", note = "© 2017 American Chemical Society. \n\nReceived: September 7, 2017; Published: November 28, 2017. \n\nSupport has been provided by the National Institutes of Health (National Institute of General Medical Sciences, grant R01-GM109194) and the Alexander von Humboldt Foundation (fellowship for J.S.). We thank Jun Myun Ahn, Bradley J. Gorsline, Dr. Paul H. Oyala (Caltech EPR Facility, supported by National Science Foundation grant NSF-1531940), Dr. Mona Shahgholi (Caltech Mass Spectrometry Facility), Dr. Yichen Tan, Dr. David G. VanderVelde (Caltech NMR Facility), and Dr. Scott C. Virgil (Caltech Center for Catalysis and Chemical Synthesis) for assistance and helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "Although the alkylation of an amine by an alkyl halide serves as a “textbook example” of a nucleophilic substitution reaction, the selective mono-alkylation of aliphatic amines by unactivated, hindered halides persists as a largely unsolved challenge in organic synthesis. We report herein that primary aliphatic amines can be cleanly mono-alkylated by unactivated secondary alkyl iodides in the presence of visible light and a copper catalyst. The method operates under mild conditions (–10 °C), displays good functional-group compatibility, and employs commercially available catalyst components. A trapping experiment with TEMPO is consistent with C–N bond formation via an alkyl radical in an out-of-cage process.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/83684, title ="Design of a Photoredox Catalyst that Enables the Direct Synthesis of Carbamate-Protected Primary Amines via Photoinduced, Copper-Catalyzed N-Alkylation Reactions of Unactivated Secondary Halides", author = "Ahn, Jun Myun and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "139", number = "49", pages = "18101-18106", month = "December", year = "2017", doi = "10.1021/jacs.7b10907", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171205-074829551", note = "© 2017 American Chemical Society. \n\nReceived: October 12, 2017; Published: December 4, 2017. \n\nSupport has been provided by the National Institutes of Health (National Institute of General Medical Sciences, grant R01-GM109194), the Natural Sciences and Engineering Research Council of Canada (graduate research fellowship for J.M.A.), the National Science Foundation (support of the Caltech EPR Facility (NSF-1531940)), and the Arnold and Mabel Beckman Foundation (support of the Beckman Institute Laser Resource Center). We thank Kareem I. Hannoun, Lawrence M. Henling, Dr. Brian C. Sanders, Dr. Jonas Schwaben, Dr. Michael K. Takase, and Dr. Haolin Yin for assistance and helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "25", abstract = "Despite the long history of S_N2 reactions between nitrogen nucleophiles and alkyl electrophiles, many such substitution reactions remain out of reach. In recent years, efforts to develop transition-metal catalysts to address this deficiency have begun to emerge. In this report, we address the challenge of coupling a carbamate nucleophile with an unactivated secondary alkyl electrophile to generate a substituted carbamate, a process that has not been achieved effectively in the absence of a catalyst; the product carbamates can serve as useful intermediates in organic synthesis as well as bioactive compounds in their own right. Through the design and synthesis of a new copper-based photoredox catalyst, bearing a tridentate carbazolide/bisphosphine ligand, that can be activated upon irradiation by blue-LED lamps, we can achieve the coupling of a range of primary carbamates with unactivated secondary alkyl bromides at room temperature. Our mechanistic observations are consistent with the new copper complex serving its intended role as a photoredox catalyst, working in conjunction with a second copper complex that mediates C–N bond formation in an out-of-cage process.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82740, title ="Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes", author = "Fajardo, Javier, Jr. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "139", number = "45", pages = "16105-16108", month = "November", year = "2017", doi = "10.1021/jacs.7b10204", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171027-140025548", note = "© 2017 American Chemical Society. \n\nReceived: September 27, 2017; Published: October 26, 2017. \n\nThis work was supported by the NIH (GM070757) and the Gordon and Betty Moore Foundation. We thank Dr. Michael K. Takase and Larry Henling for crystallographic assistance. J.F.J. acknowledges support of the NSF for a Graduate Fellowship (GRFP). \n\nThe authors declare no competing financial interest.", revision_no = "29", abstract = "Despite the critical role Ru and Os complexes have played in the development of transition metal dinitrogen chemistry, they have not been shown to mediate catalytic N_2-to-NH_3 conversion (N_2RR), nor have M-N_xH_y complexes been derived from protonation of their M-N_2 precursors. To help delineate factors for N_2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N_2RR, and compare their activities with an isostructural Fe complex. The Os system is most active, and liberates more than 120 equiv NH_3 per Os center in a single batch experiment using Cp*_2Co and [H_2NPh_2][OTf] as reductant and acid source. Isostructural Ru and Fe complexes generate little NH_3 under the same conditions. Protonation of Os-N_2– affords a structurally characterized Os=NNH_2+ hydrazido species that mediates NH_3 generation, suggesting it is a plausible intermediate of the catalysis. Inactive Os hydrides are characterized that form during catalysis.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/82248, title ="Nitrogen Fixation via a Terminal Fe(IV) Nitride", author = "Thompson, Niklas B. and Green, Michael T.", journal = "Journal of the American Chemical Society", volume = "139", number = "43", pages = "15312-15315", month = "November", year = "2017", doi = "10.1021/jacs.7b09364", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20171010-095936880", note = "© 2017 American Chemical Society. \n\nReceived: September 1, 2017; Published: October 9, 2017. \n\nWe thank M. Latimer, E. Nelson, C. Krest, C. Miller, and K. Mittra for assistance with synchrotron measurements. This work was supported by the Resnick Sustainability Institute at Caltech (Graduate Fellowship, N.B.T.), as well as the NIH (GM 070757). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Additional notes in the Supporting Information. \n\nThe authors declare no competing financial interest.", revision_no = "29", abstract = "Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N_2. Here we show that a nitrogen-fixing Fe–N_2 catalyst can be protonated to form a neutral Fe(NNH_2) hydrazido(2−) intermediate, which, upon further protonation, heterolytically cleaves the N–N bond to release [Fe^(IV)≡N]^+ and NH_3. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N_2-to-NH_3 conversion. The physical oxidation state range of the Fe complexes in this transformation is buffered by covalency with the ligand, a feature of possible relevance to catalyst design in synthetic and natural systems that facilitate multiproton/multielectron redox processes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/80616, title ="Photoinduced, Copper-Catalyzed Alkylation of Amines: A Mechanistic Study of the Cross-Coupling of Carbazole with Alkyl Bromides", author = "Ahn, Jun Myun and Ratani, Tanvi S.", journal = "Journal of the American Chemical Society", volume = "139", number = "36", pages = "12716-12723", month = "September", year = "2017", doi = "10.1021/jacs.7b07052", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170818-105931485", note = "© 2017 American Chemical Society. \n\nReceived: July 6, 2017; Published: August 17, 2017. \n\nSupport has been provided by the National Institutes of Health (National Institute of General Medical Sciences: R01–109194), the Natural Sciences and Engineering Research Council of Canada (graduate research fellowship for J.M.A.), and the National Science Foundation (support of the Caltech EPR Facility (NSF-1531940) and a graduate research fellowship for T.S.R.). Additional support has been provided by the Arnold and Mabel Beckman Foundation through the Caltech Beckman Institute Laser Resource Center. We thank Dr. Angel J. Di Bilio, Dr. Paul Oyala, Dr. Sidney E. Creutz, Lawrence M. Henling, Dr. Marcin Kalek, Dr. Wesley Sattler, Dr. Oliver S. Shafaat, Dr. Mona Shahgholi, Dr. David VanderVelde, and Dr. Jay R. Winkler for technical assistance and helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "29", abstract = "We have recently reported that a variety of couplings of nitrogen, sulfur, oxygen, and carbon nucleophiles with organic halides can be achieved under mild conditions (−40 to 30 °C) through the use of light and a copper catalyst. Insight into the various mechanisms by which these reactions proceed may enhance our understanding of chemical reactivity and facilitate the development of new methods. In this report, we apply an array of tools (EPR, NMR, transient absorption, and UV–vis spectroscopy; ESI–MS; X-ray crystallography; DFT calculations; reactivity, stereochemical, and product studies) to investigate the photoinduced, copper-catalyzed coupling of carbazole with alkyl bromides. Our observations are consistent with pathways wherein both an excited state of the copper(I) carbazolide complex ([Cu^I(carb)_2]^−) and an excited state of the nucleophile (Li(carb)) can serve as photoreductants of the alkyl bromide. The catalytically dominant pathway proceeds from the excited state of Li(carb), generating a carbazyl radical and an alkyl radical. The cross-coupling of these radicals is catalyzed by copper via an out-of-cage mechanism in which [Cu^I(carb)_2]^− and [Cu^(II)(carb)_3]^− (carb = carbazolide), both of which have been identified under coupling conditions, are key intermediates, and [Cu^(II)(carb)_3]^− serves as the persistent radical that is responsible for predominant cross-coupling. This study underscores the versatility of copper(II) complexes in engaging with radical intermediates that are generated by disparate pathways, en route to targeted bond constructions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/80847, title ="Photoinduced, Copper-Catalyzed Decarboxylative C–N Coupling to Generate Protected Amines: An Alternative to the Curtius Rearrangement", author = "Zhao, Wei and Wurz, Ryan P.", journal = "Journal of the American Chemical Society", volume = "139", number = "35", pages = "12153-12156", month = "September", year = "2017", doi = "10.1021/jacs.7b07546", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170828-124616088", note = "© 2017 American Chemical Society. \n\nReceived: July 19, 2017; Published: August 25, 2017. \n\nSupport has been provided by Amgen and by the National Institutes of Health (NIGMS: R01-109194). We thank Jun Myun Ahn, Dr. Victor J. Cee, Bradley J. Gorsline, Carson D. Matier, Dr. Paul H. Oyala (Caltech EPR facility, supported by NSF-1531940), and Dr. Haolin Yin for assistance and helpful discussions. \n\nThe authors declare no competing financial interest.", revision_no = "23", abstract = "The Curtius rearrangement is a classic, powerful method for converting carboxylic acids into protected amines, but its widespread use is impeded by safety issues (the need to handle azides). We have developed an alternative to the Curtius rearrangement that employs a copper catalyst in combination with blue-LED irradiation to achieve the decarboxylative coupling of aliphatic carboxylic acid derivatives (specifically, readily available N-hydroxyphthalimide esters) to afford protected amines under mild conditions. This C–N bond-forming process is compatible with a wide array of functional groups, including an alcohol, aldehyde, epoxide, indole, nitroalkane, and sulfide. Control reactions and mechanistic studies are consistent with the hypothesis that copper species are engaged in both the photochemistry and the key bond-forming step, which occurs through out-of-cage coupling of an alkyl radical.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/79307, title ="CO_2 Reduction Selective for C_(≥2) Products on Polycrystalline Copper with N-Substituted Pyridinium Additives", author = "Han, Zhiji and Kortlever, Ruud", journal = "ACS Central Science", volume = "3", number = "8", pages = "853-859", month = "August", year = "2017", doi = "10.1021/acscentsci.7b00180", issn = "2374-7943", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170724-131758454", note = "© 2017 American Chemical Society. ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. \n\nReceived: April 26, 2017; Published: July 21, 2017. \n\nNMR and XPS spectra were collected at the NMR Facility (Division of CCE) and Molecular Materials Research Center (Beckman Institute) of the California Institute of Technology, respectively. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "Electrocatalytic CO_2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO_2 reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH_4 and H_2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO_2 reduction to C_(≥2) hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C_2 and C_3 products with a hydrocarbon ratio of C_(≥2)/CH4significantly greater than 100 have been observed with several additives. ^(13)C-labeling studies verify CO_2 to be the sole carbon source in the C_(≥2) hydrocarbons produced. Upon electroreduction, the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C_(≥2) species to H_2 (∼90%) while suppressing methane with certain N-heterocyclic additives.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/77446, title ="N_2-to-NH_3 Conversion by a triphos-Iron Catalyst and Enhanced Turnover under Photolysis", author = "Buscagan, Trixia M. and Oyala, Paul H.", journal = "Angewandte Chemie International Edition", volume = "56", number = "24", pages = "6921-6926", month = "June", year = "2017", doi = "10.1002/anie.201703244", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170515-102546782", note = "© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nManuscript received: March 29, 2017; Version of record online: May 10, 2017.\n\nThis work was supported by the NIH (GM070757 and a Ruth\u2005L. Kirschstein NRSA Predoctoral Fellowship to Promote Diversity in Health-Related Research to T.M.B.), and the NSF via its MRI program (NSF-1531940). We thank Lawrence Henling and Michael Takase for assistance with XRD studies, Javier Fajardo,\u2005Jr., Matthew Chalkley, and Niklas Thompson for insightful discussions, and Dr. Shabnam Hematian for assistance with GC experiments. \n\nThe authors declare no conflict of interest.", revision_no = "22", abstract = "Bridging iron hydrides are proposed to form at the active site of MoFe-nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N_2 via reductive elimination of H_2. This possibility inspires the investigation of well-defined molecular iron hydrides as precursors for catalytic N_2-to-NH_3 conversion. Herein, we describe the synthesis and characterization of new P_2^(P′Ph)Fe(N_2)(H)_x systems that are active for catalytic N_2-to-NH_3 conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that photo-elimination of H_2 is one means by which the enhanced activity may arise.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/74426, title ="Catalytic N_2-to-NH_3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET", author = "Chalkley, Matthew J. and Del Castillo, Trevor J.", journal = "ACS Central Science", volume = "3", number = "3", pages = "217-223", month = "March", year = "2017", doi = "10.1021/acscentsci.7b00014", issn = "2374-7943", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170221-122959469", note = "© 2017 American Chemical Society. ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. \n\nReceived: January 9, 2017. Publication Date (Web): February 14, 2017. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1053575. M.J.C., T.J.D.C., and B.D.M. acknowledge the support of the NSF for Graduate Fellowships (GRFP). \n\nAuthor Contributions: M.J.C., T.J.D.C., and B.D.M. contributed equally to this work. \n\nThe authors declare no competing financial interest.", revision_no = "24", abstract = "We have recently reported on several Fe catalysts for N_2-to-NH_3 conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC_8) and acid ([H(OEt_2)_2][BArF_4]). Here we show that our original catalyst system, P_3^BFe, achieves both significantly improved efficiency for NH_3 formation (up to 72% for e^– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH_3 per Fe site), when employing a significantly weaker combination of reductant (Cp*_2Co) and acid ([Ph_2NH_2][OTf] or [PhNH_3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/73060, title ="Exploring secondary-sphere interactions in Fe–N_xH_y complexes relevant to N_2 fixation", author = "Creutz, Sidney E. and Peters, Jonas C.", journal = "Chemical Science", volume = "8", number = "3", pages = "2321-2328", month = "March", year = "2017", doi = "10.1039/c6sc04805f", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20161221-100943169", note = "© 2016 The Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. \n\nReceived 31st October 2016; Accepted 7th December 2016; First published online 08 Dec 2016. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. Larry Henling and Mike Takase are thanked for crystallographic assistance.", revision_no = "21", abstract = "Hydrogen bonding and other types of secondary-sphere interactions are ubiquitous in metalloenzyme active sites and are critical to the transformations they mediate. Exploiting secondary sphere interactions in synthetic catalysts to study the role(s) they might play in biological systems, and to develop increasingly efficient catalysts, is an important challenge. Whereas model studies in this broad context are increasingly abundant, as yet there has been relatively little progress in the area of synthetic catalysts for nitrogen fixation that incorporate secondary sphere design elements. Herein we present our first study of Fe–NxHy complexes supported by new tris(phosphine)silyl ligands, abbreviated as [SiP^(Nme_3)] and [SiP^(iPr_2)P^(Nme)], that incorporate remote tertiary amine hydrogen-bond acceptors within a tertiary phosphine/amine 6-membered ring. These remote amine sites facilitate hydrogen-bonding interactions via a boat conformation of the 6-membered ring when certain nitrogenous substrates (e.g., NH_3 and N_2H_4) are coordinated to the apical site of a trigonal bipyramidal iron complex, and adopt a chair conformation when no H-bonding is possible (e.g., N_2). Countercation binding at the cyclic amine is also observed for anionic {Fe–N_2}− complexes. Reactivity studies in the presence of proton/electron sources show that the incorporated amine functionality leads to rapid generation of catalytically inactive Fe–H species, thereby substantiating a hydride termination pathway that we have previously proposed deactivates catalysts of the type [EP^R_3]FeN_2 (E = Si, C).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/73916, title ="N-H Bond Dissociation Enthalpies and Facile H-atom Transfers for Early Intermediates of Fe-N_2 and Fe-CN Reductions", author = "Rittle, Jonathan and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "139", number = "8", pages = "3161-3170", month = "March", year = "2017", doi = "10.1021/jacs.6b12861", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170201-092020177", note = "© 2017 American Chemical Society. \n\nReceived: December 14, 2016; Published: January 31, 2017. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. JR was additionally supported by a fellowship from the Caltech Center for Environmental Microbial Interactions (CEMI).", revision_no = "35", abstract = "Fe-mediated biological nitrogen fixation is thought to proceed either via a sequence of proton and electron transfer steps, concerted H-atom transfer steps, or some combination thereof. Regardless of the specifics, and whether the intimate mechanism for N_2-to-NH_3 conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-N_xH_y intermediates are implicated that feature reactive N-H bonds. Thermodynamic knowledge of the N-H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe-N=NH, Fe=N-NH_2, Fe-NH=NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation, and to the rational design of improved synthetic N_2 fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N_2 and Fe-CN species at the terminal N-atom (e.g., Fe=N-NH_2, Fe-CNH, FeC-NH_2). These same Fe-N_2 and Fe-CN systems are functionally active for N_2-to-NH_3 and CN-to-CH_4/NH_3 conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N-H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N-H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N-H bonds, and estimate their homolytic N-H bond enthalpies via redox and acidity titrations. Very low N-H bond dissociation enthalpies (BDE_(N-H)), ranging from 65 (e.g., Fe-CNH) to ≤ 37 kcal/mol (Fe-N=NH), are determined. The collective data presented herein provides insight into the facile reactivity profiles of early stage protonated Fe-N_2 and Fe-CN species.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/74242, title ="CO Reduction to CH_3OSiMe_3: Electrophile-Promoted Hydride Migration at a Single Fe Site", author = "Deegan, Meaghan M. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "139", number = "7", pages = "2561-2564", month = "February", year = "2017", doi = "10.1021/jacs.6b12444", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170213-121746843", note = "© 2017 American Chemical Society. \n\nReceived: December 2, 2016; Published: February 3, 2017. \n\nThis work was supported by the NIH (GM070757) and the Gordon and Betty Moore Foundation. We thank Larry Henling and Mike Takase for assistance with crystallography. \n\nThe authors declare no competing financial interest.", revision_no = "26", abstract = "One of the major challenges associated with developing molecular Fischer–Tropsch catalysts is the design of systems that promote the formation of C–H bonds from H_2 and CO while also facilitating the release of the resulting CO-derived organic products. To this end, we describe the synthesis of reduced iron-hydride/carbonyl complexes that enable an electrophile-promoted hydride migration process, resulting in the reduction of coordinated CO to a siloxymethyl (LnFe-CH_2OSiMe_3) group. Intramolecular hydride-to-CO migrations are extremely rare, and to our knowledge the system described herein is the first example where such a process can be accessed from a thermally stable M(CO)(H) complex. Further addition of H_2 to LnFe-CH_2OSiMe_3 releases CH_3OSiMe_3, demonstrating net four-electron reduction of CO to CH_3OSiMe_3 at a single Fe site.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/72691, title ="Gastight Hydrodynamic Electrochemistry: Design for a Hermetically Sealed Rotating Disk Electrode Cell", author = "Jung, Suho and Kortlever, Ruud", journal = "Analytical Chemistry", volume = "89", number = "1", pages = "581-585", month = "January", year = "2017", doi = "10.1021/acs.analchem.6b04228", issn = "0003-2700", url = "https://resolver.caltech.edu/CaltechAUTHORS:20161209-102641774", note = "© 2016 American Chemical Society. \n\nReceived: October 28, 2016; Accepted: December 7, 2016; Published: December 7, 2016. \n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. \n\nThe authors declare no competing financial interest.", revision_no = "20", abstract = "Rotating disk electrodes (RDEs) are widely used in electrochemical characterization to analyze the mechanisms of various electrocatalytic reactions. RDE experiments often make use of or require collection and quantification of gaseous products. The combination of rotating parts and gaseous analytes makes the design of RDE cells that allow for headspace analysis challenging due to gas leaks at the interface of the cell body and the rotator. In this manuscript we describe a new, hermetically-sealed electrochemical cell that allows for electrode rotation while simultaneously providing a gastight environment. Electrode rotation in this new cell design is controlled by magnetically coupling the working electrode to a rotating magnetic driver. Calibration of the RDE using a tachometer shows that the rotation speed of the electrode is the same as that of the magnetic driver. To validate the performance of this cell for hydrodynamic measurements, limiting currents from the reduction of a potassium ferrocyanide (K_4[Fe(CN)_6] •3H_2O) were measured and shown to compare favorably with calculated values from the Levich equation and with data obtained using more typical, non-gastight RDE cells. Faradaic efficiencies of ~95% were measured in the gas phase for oxygen evolution in alkaline media at an Inconel 625 alloy electrocatalyst during rotation at 1600 rpm. These data verify that a gastight environment is maintained even during rotation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/70271, title ="Proton-Coupled Reduction of an Iron Cyanide Complex to Methane and Ammonia", author = "Rittle, Jonathan and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "55", number = "40", pages = "12262-12265", month = "September", year = "2016", doi = "10.1002/anie.201606366", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160912-090559386", note = "© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nReceived: June 30, 2016; First published: 8 September 2016. \n\nThis work was supported by the NIH (grant number GM 070757) and the NSF (GRFP to J.R.). We thank Lawrence Henling and Michael Takase for assistance with XRD studies, and Kathryn Perez and Nathan Dalleska for assistance with GC experiments.", revision_no = "31", abstract = "Nitrogenase enzymes mediate the six-electron reductive cleavage of cyanide to CH_4 and NH_3. Herein we demonstrate for the first time the liberation of CH_4 and NH_3 from a well-defined iron cyanide coordination complex, [SiP^(iPr)_3]Fe(CN) (where [SiP^(iPr)_3] represents a tris(phosphine)silyl ligand), on exposure to proton and electron equivalents. [SiP^(iPr)_3]Fe(CN) additionally serves as a useful entry point to rare examples of terminally-bound Fe(CNH) and Fe(CNH_2) species that, in accord with preliminary mechanistic studies, are plausible intermediates of the cyanide reductive protonation to generate CH_4 and NH_3. Comparative studies with a related [SiP^(iPr)_3]Fe(CNMe_2) complex suggests the possibility of multiple, competing mechanisms for cyanide activation and reduction.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69973, title ="A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}^(8-10)", author = "Chalkley, Matthew J. and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "55", number = "39", pages = "11995-11998", month = "September", year = "2016", doi = "10.1002/anie.201605403", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160826-114805360", note = "© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nReceived: June 2, 2016; Revised: July 10, 2016; First published: 25 August 2016. \n\nThis research was supported by the NIH (GM-070757) and an NSF Graduate Research Fellowship to M.J.C. We thank Larry Henling and Dr. Michael K. Takase for crystallographic assistance. We acknowledge Dr. Gaël Ung for preliminary data on the {TPBFeNO}^8 complex and helpful discussions.", revision_no = "27", abstract = "Given the importance of Fe–NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark–Feltham numbers: {FeNO}^8, {FeNO}^9, and {FeNO}^(10). These {FeNO}^(8–10) compounds are each low-spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}^(8–10) are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark–Feltham numbers.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/69808, title ="Breaking the Correlation between Energy Costs and Kinetic Barriers in Hydrogen Evolution via a Cobalt Pyridine-Diimine-Dioxime Catalyst", author = "Huo, Pengfei and Uyeda, Christopher", journal = "ACS Catalysis", volume = "6", number = "9", pages = "6114-6123", month = "September", year = "2016", doi = "10.1021/acscatal.6b01387", issn = "2155-5435", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160822-125634395", note = "© 2016 American Chemical Society. \n\nReceived: May 17, 2016; Revised: July 18, 2016; Publication Date (Web): July 26, 2016. \n\nThis work was supported by the Air Force Office of Scientific Research (USAFOSR) under Grant No. FA9550-11-1-0288, the (U.S.) Department of Energy (DOE) under Grant No. DESC0006598, and by the NSF Center for Chemical Innovation Solar Fuels Grant CHE-1305124. P.H. thanks Kara Bren and Richard Eisenberg for helpful discussions. Computing resources were provided by the National Energy Research Scientific Computing Center (NERSC) (DE-AC02-05CH11231) and XSEDE (TG-CHE130108). \n\nThe authors declare no competing financial interest.", revision_no = "25", abstract = "A central challenge in the development of inorganic hydrogen evolution catalysts is to avoid deleterious coupling between the energetics of metal site reduction and the kinetics of metal hydride formation. In this work, we combine theoretical and experimental methods to investigate cobalt diimine-dioxime catalysts that show promise for achieving this aim by introducing an intramolecular proton shuttle via a pyridyl pendant group. Using over 200 coupled-cluster-level electronic structure calculations of the Co-based catalyst with a variety of pyridyl substituents, the energetic and kinetic barriers to hydrogen formation are investigated, revealing nearly complete decoupling of the energetics of Co reduction and the kinetics of intramolecular Co hydride formation. These calculations employ recently developed quantum embedding methods that allow for local regions of a molecule to be described using high-accuracy wavefunction methods (such as CCSD(T)), thus overcoming significant errors in the DFT-level description of transition-metal complexes. Experimental synthesis and cyclic voltammetry of the methyl-substituted form of the catalyst indicate that protonation of the pendant group leaves the Co reduction potential unchanged, which is consistent with the theoretical prediction that these catalysts can successfully decouple the electronic structures of the transition-metal and ligand-protonation sites. Additional computational analysis indicates that introduction of the pyridyl pendant group enhances the favorability of intramolecular proton shuttling in these catalysts by significantly reducing the energetic barrier for metal hydride formation relative to previously studied cobalt diimine-dioxime catalysts. These results demonstrate a promising proof of principle for achieving uncoupled and locally tunable intramolecular charge-transfer events in the context of homogeneous transition-metal catalysts.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/67579, title ="Spin-State Tuning at Pseudo-tetrahedral d^6 Ions: Spin Crossover in [BP_3]Fe^(II)–X Complexes", author = "Creutz, Sidney E. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "55", number = "8", pages = "3894-3906", month = "April", year = "2016", doi = "10.1021/acs.inorgchem.6b00066", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160602-132307484", note = "© 2016 American Chemical Society. \n\nReceived: January 10, 2016. Published: April 4, 2016. Publication Date (Web): April 04, 2016. \n\nThis work was supported by the Gordon and Betty Moore Foundation and through the NSF via a GRFP award to S.E.C. Larry Henling and Michael Takase are thanked for their assistance with X-ray crystallography. The Molecular Materials Research Center (MMRC) at Caltech is thanked for the use of the SQUID magnetometer. \n\nAuthor Contributions: The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. \n\nThe authors declare no competing financial interest.", revision_no = "16", abstract = "Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series of four-coordinate, pseudo-tetrahedral P_3Fe^(II)–X complexes supported by tris(phosphine)borate P_3 ([PhBP_3^R]^−) and phosphiniminato X-type ligands (−N=PR3′) that, in combination, tune the spin-crossover behavior of the system. Most of the reported iron complexes undergo spin crossover at temperatures near or above room temperature in solution and in the solid state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the ultraviolet–visible (UV-vis) absorption spectra, which allows for quantitative modeling of the thermodynamic parameters of the spin equilibria. These spin equilibria have also been studied by numerous techniques including paramagnetic nuclear magnetic resonance (NMR), infrared, and Mössbauer spectroscopies; X-ray crystallography; and solid-state superconducting quantum interference device (SQUID) magnetometry. These studies allow qualitative correlations to be made between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes associated with the spin equilibria.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/65712, title ="An Fe-N_2 Complex That Generates Hydrazine and Ammonia via Fe═NNH_2: Demonstrating a Hybrid Distal-to-Alternating Pathway for N_2 Reduction", author = "Rittle, Jonathan and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "138", number = "12", pages = "4243-4248", month = "March", year = "2016", doi = "10.1021/jacs.6b01230", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160328-143527690", note = "© 2016 American Chemical Society. \n\nReceived: February 2, 2016; Publication Date (Web): March 03, 2016. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. J.R. was additionally supported by a fellowship from the Caltech Center for Environmental Microbial Interactions (CEMI). \n\nThe authors declare no competing financial interest.", revision_no = "28", abstract = "Biological N_2 fixation to NH_3 may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e–/H+ transfer steps of iron-ligated N_2 in well-defined synthetic systems is hence of much interest but remains a significant challenge. While iron complexes have been recently discovered that catalyze the formation of NH_3 from N_2, mechanistic details remain uncertain. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate Fe═NNH_2+ species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N_2– complex. The Fe═NNH_2+ complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = 1/2 Fe═NNH_2. At warmer temperatures, Fe═NNH_2 rapidly converts to an iron hydrazine complex, Fe-NH_2NH_2+, via the additional transfer of proton and electron equivalents in solution. Fe-NH_2NH_2+ can liberate NH_3, and the sequence of reactions described here hence demonstrates that an iron site can shuttle from a distal intermediate (Fe═NNH_2+) to an alternating intermediate (Fe-NH_2NH_2+) en route to NH_3 liberation from N_2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N_2 reduction may be operative in biological N_2 fixation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/66312, title ="A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench ^(57)Fe Mössbauer Data, and a Hydride Resting State", author = "Del Castillo, Trevor J. and Thompson, Niklas B.", journal = "Journal of the American Chemical Society", volume = "138", number = "16", pages = "5341-5350", month = "March", year = "2016", doi = "10.1021/jacs.6b01706", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160420-105550849", note = "© 2016 American Chemical Society. \n\nReceived: February 16, 2016; Publication Date (Web): March 29, 2016. \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. T.J.D.C. acknowledges the support of the NSF for a Graduate Fellowship (GRFP), and N.B.T. acknowledges the support of the Resnick Sustainability Institute at Caltech for a Graduate Fellowship. \n\nT.J.D.C. and N.B.T. contributed equally to this work. \n\nThe authors declare no competing financial interest.", revision_no = "27", abstract = "The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe–N_2 complexes of tetradentate P_3^E ligands (E = B, C) generate catalytic yields of NH_3 under an atmosphere of N_2 with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of magnitude improvement in yield of NH_3 for each Fe catalyst has been realized (up to 64 equiv of NH_3 produced per Fe for P_3^B and up to 47 equiv for P_3^C) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P_3^BFe–N_2/P_3^BFe–N_2– couple and controlled-potential electrolysis of P_3^BFe^+ at −45 °C demonstrates that electrolytic N_2 reduction to NH_3 is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P_3^B), consistent with a single-site catalyst model. An isostructural system (P_3^(Si)) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-quench Mössbauer spectroscopy during turnover reveals an iron–borohydrido–hydride complex as a likely resting state of the P_3^BFe catalyst system. We postulate that hydrogen-evolving reaction activity may prevent iron hydride formation from poisoning the P_3^BFe system. This idea may be important to consider in the design of synthetic nitrogenases and may also have broader significance given that intermediate metal hydrides and hydrogen evolution may play a key role in biological nitrogen fixation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/64908, title ="A Ni^0(η^2-(Si–H))(η^2-H_2) Complex That Mediates Facile H Atom Exchange between Two σ-Ligands", author = "Connor, Bridget A. and Rittle, Jonathan", journal = "Organometallics", volume = "35", number = "5", pages = "686-690", month = "March", year = "2016", doi = "10.1021/acs.organomet.5b00985", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160301-114837187", note = "© 2016 American Chemical Society. \n\nReceived: December 2, 2015;\nPublished: February 22, 2016.\n\nThis work was supported by the NSF Center for Chemical Innovation Solar Fuels Grant CHE-1305124. We also acknowledge the Gordon and Betty Moore Foundation for financial support. B.A.C. acknowledges an Arthur R. Adams SURF fellowship. \n\nThe authors declare no competing financial interest.", revision_no = "28", abstract = "σ-adduct complexes of low-valent, late first-row metal complexes are highly unusual, and this is particularly true of d^(10) systems. We have discovered a nickel/phosphine/silyl system that undergoes reaction with H_2 in solution to generate a species best described as Ni^0(η^2-(Si–H))(η^2-H_2) on the basis of multinuclear NMR studies. Theoretical calculations suggest that the Ni center facilitates H atom exchange between the η^2-(Si–H) and η^2-H_2 ligands via interconversion with a higher valent Ni^(II) isomer. This exchange is exploited in the selective, catalytic deuteration of exogenous silanes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/62931, title ="Benchmarking nanoparticulate metal oxide electrocatalysts for the alkaline water oxidation reaction", author = "Jung, Suho and McCrory, Charles C. L.", journal = "Journal of Materials Chemistry A", volume = "4", number = "8", pages = "3068-3076", month = "February", year = "2016", doi = "10.1039/c5ta07586f", issn = "2050-7488", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151215-102451419", note = "© 2015 The Royal Society of Chemistry. \n\nReceived 22nd September 2015; Accepted 25th November 2015; First published online 27 Nov 2015. \n\nThis article is part of themed collection: Water splitting and photocatalysis. \n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. We would like to acknowledge much assistance in BET measurements by Kurt M. Van Allsburg.", revision_no = "20", abstract = "Nanoparticulate metal-oxide catalysts are among the most prevalent systems for alkaline water oxidation. However, comparisons of the electrochemical performance of these materials have been challenging due to the different methods of attachment, catalyst loadings, and electrochemical test conditions reported in the literature. Herein, we have leveraged a conventional drop-casting method that allows for the successful adhesion of a wide range of nanoparticulate catalysts to glassy-carbon electrode surfaces. We have applied this adhesion method to prepare catalyst films from 16 crystalline metal-oxide nanoparticles with a constant loading of 0.8 mg cm^(−2), and evaluated the resulting nanoparticulate films for the oxygen evolution reaction under conditions relevant to an integrated solar fuels device. In general, the activities of the adhered nanoparticulate films are similar to those of thin-film catalysts prepared by electrodeposition or sputtering, achieving 10 mA cm^(−2) current densities per geometric area at overpotentials of ∼0.35–0.5 V.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/65627, title ="A mechanistic investigation of the photoinduced, copper-mediated cross-coupling of an aryl thiol with an aryl halide", author = "Johnson, Miles W. and Hannoun, Kareem I.", journal = "Chemical Science", volume = "7", pages = "4091-4100", month = "February", year = "2016", doi = "10.1039/c5sc04709a", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160323-130838018", note = "© 2016 The Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. \n\nReceived 07 Dec 2015, Accepted 16 Feb 2016, First published online 24 Feb 2016. \n\nWe thank Jun Myun Ahn, Dr Angel J. Di Bilio, Lawrence M. Henling, Tanvi S. Ratani, Dr David VanderVelde, Dr Jay R. Winkler, and members of the laboratory of Prof. Harry B. Gray for assistance, and we thank the Gordon and Betty Moore Foundation and the NIH (NIGMS: R01 GM109194) for funding. Photophysical studies were conducted at the Beckman Institute Laser Resource Center. M. W. J. would like to acknowledge the National Institute of Health for a Ruth L. Kirschstein National Research Service Award (F32GM116412).", revision_no = "26", abstract = "Photoinduced, copper-catalyzed cross-coupling can offer a complementary approach to thermal (non-photoinduced) methods for generating C–X (X = C, N, O, S, etc.) bonds. In this report, we describe the first detailed mechanistic investigation of one of the processes that we have developed, specifically, the (stoichiometric) coupling of a copper–thiolate with an aryl iodide. In particular, we focus on the chemistry of a discrete [Cu^I(SAr)_2]− complex (Ar = 2,6-dimethylphenyl), applying a range of techniques, including ESI-MS, cyclic voltammetry, transient luminescence spectroscopy, optical spectroscopy, DFT calculations, Stern–Volmer analysis, EPR spectroscopy, actinometry, and reactivity studies. The available data are consistent with the viability of a pathway in which photoexcited [Cu^I(SAr)_2]−* serves as an electron donor to an aryl iodide to afford an aryl radical, which then reacts in cage with the newly generated copper(II)–thiolate to furnish the cross-coupling product in a non-chain process.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/64493, title ="Asymmetric copper-catalyzed C-N cross-couplings induced by visible light", author = "Kainz, Quirin M. and Matier, Carson D.", journal = "Science", volume = "351", number = "6274", pages = "681-684", month = "February", year = "2016", doi = "10.1126/science.aad8313", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20160216-092106614", note = "© 2016 American Association for the Advancement of Science. \n\nReceived 10 November 2015; accepted 7 January 2016. \n\nSupport has been provided by NIH (National Institute of General Medical Sciences, grant R01–GM109194), the Gordon and Betty Moore Foundation, the Alexander von Humboldt Foundation (fellowship for Q.M.K.), and the Bengt Lundqvist Memorial Foundation of the Swedish Chemical Society (fellowship for A.B.). We thank J. M. Ahn, L. M. Henling (Caltech X-Ray Crystallography Facility), M. W. Johnson, N. D. Schley, M. Shahgholi (Caltech Mass Spectrometry Facility), M. K. Takase (Caltech X-Ray Crystallography Facility), N. Torian (Caltech Mass Spectrometry Facility), D. G. VanderVelde (Caltech NMR Facility), and S. C. Virgil (Caltech Center for Catalysis and Chemical Synthesis) for assistance and helpful discussions. Experimental procedures and characterization data are provided in the supplementary materials. Metrical parameters for the structures of compounds 1 to 4 are available free of charge from the Cambridge Crystallographic Data Centre under accession numbers CCDC 1435979, 1435978, 1435977, and 1435980.", revision_no = "14", abstract = "Despite a well-developed and growing body of work in copper catalysis, the potential of copper to serve as a photocatalyst remains underexplored. Here we describe a photoinduced copper-catalyzed method for coupling readily available racemic tertiary alkyl chloride electrophiles with amines to generate fully substituted stereocenters with high enantioselectivity. The reaction proceeds at –40°C under excitation by a blue light-emitting diode and benefits from the use of a single, Earth-abundant transition metal acting as both the photocatalyst and the source of asymmetric induction. An enantioconvergent mechanism transforms the racemic starting material into a single product enantiomer.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/60975, title ="Evaluating Activity for Hydrogen-Evolving Cobalt and Nickel Complexes at Elevated Pressures of Hydrogen and Carbon Monoxide", author = "McCrory, Charles C. L. and Szymczak, Nathaniel K.", journal = "Electrocatalysis", volume = "7", number = "1", pages = "87-96", month = "January", year = "2016", doi = "10.1007/s12678-015-0281-y", issn = "1868-2529", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151012-110949195", note = "© 2015 Springer Science+Business Media New York. \n\nFirst online: 05 October 2015. \n\nFinancial support for this work was provided by an NSF Center for Chemical Innovation (CHE-0802907). We also thank David C. Lacy and Wesley K. Kramer for many useful discussions.", revision_no = "16", abstract = "Molecular cobalt and nickel complexes are among the most promising homogeneous systems for electrocatalytic hydrogen evolution. However, there has been little exploration into the effect of gaseous co-additives such as CO and H_2, which may be present in operating hydrogen-evolving or carbon-dioxide reduction systems, on the performance of these molecular electrocatalysts. In this report, we investigate the electrocatalytic activity of six cobalt and nickel complexes supported by tetraazamacrocyclic or diazadiphosphacyclooctane ligands for the reduction of p-toluenesulfonic acid to hydrogen in acetonitrile under inert atmosphere and in the presence of CO and H_2. We present an elevated-pressure electrochemical apparatus capable of reaching CO and H_2 pressures of ca. 15–520 pounds per square inch (psia) (∼1–35 atm), and we use this apparatus to determine binding constants for CO addition for each catalyst and study the inhibition of the electrocatalysis as a function of CO and H_2 pressure. In the case of CO, the extent of catalytic inhibition is correlated to the binding constant, with the cobalt complexes showing a greater degree of catalyst inhibition compared to the nickel complexes. In the case of H2, no complex showed appreciable electrocatalytic inhibition even at H_2 pressures of ca. 500 psia.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61560, title ="Photoinduced, Copper-Catalyzed Carbon–Carbon Bond Formation with Alkyl Electrophiles: Cyanation of Unactivated Secondary Alkyl Chlorides at Room Temperature", author = "Ratani, Tanvi S. and Bachman, Shoshana", journal = "Journal of the American Chemical Society", volume = "137", number = "43", pages = "13902-13907", month = "November", year = "2015", doi = "10.1021/jacs.5b08452", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151027-113729482", note = "© 2015 American Chemical Society. \n\nReceived: August 10, 2015. Publication Date (Web): October 22, 2015. \n\nWe thank Dr. Junwon Choi, Nathaniel T. Kadunce, Dr. Wesley Sattler, Dr. David VanderVelde, Dr. Scott C. Virgil, Paul Walton, and Dr. Daniel T. Ziegler for experimental assistance and for helpful discussions, and we thank the Gordon and Betty Moore Foundation, the National Science Foundation (graduate research fellowship to T.S.R.), and the NIH (NIGMS: R01 GM109194) for funding. \n\nT.S.R. and S.B. contributed equally to this work. \n\nThe authors declare no competing financial interest.", revision_no = "31", abstract = "We have recently reported that, in the presence of light and a copper catalyst, nitrogen nucleophiles such as carbazoles and primary amides undergo C–N coupling with alkyl halides under mild conditions. In the present study, we establish that photoinduced, copper-catalyzed alkylation can also be applied to C–C bond formation, specifically, that the cyanation of unactivated secondary alkyl chlorides can be achieved at room temperature to afford nitriles, an important class of target molecules. Thus, in the presence of an inexpensive copper catalyst (CuI; no ligand coadditive) and a readily available light source (UVC compact fluorescent light bulb), a wide array of alkyl halides undergo cyanation in good yield. Our initial mechanistic studies are consistent with the hypothesis that an excited state of [Cu(CN)_2]^− may play a role, via single electron transfer, in this process. This investigation provides a rare example of a transition metal-catalyzed cyanation of an alkyl halide, as well as the first illustrations of photoinduced, copper-catalyzed alkylation with either a carbon nucleophile or a secondary alkyl chloride.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/60126, title ="E–H Bond Activations and Hydrosilylation Catalysis with Iron and Cobalt Metalloboranes", author = "Nesbit, Mark A. and Suess, Daniel L. M.", journal = "Organometallics", volume = "34", number = "19", pages = "4741-4752", month = "October", year = "2015", doi = "10.1021/acs.organomet.5b00530", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150909-111957640", note = "© 2015 American Chemical Society. \n\nReceived: June 18, 2015; Publication Date (Web): August 28, 2015. \n\nThe authors would like to acknowledge the financial support provided by the NSF Center for Chemical Innovation (CCI; CHE-1305124) and the Gordon and Betty Moore Foundation. The authors thank Dr. Michael Takase and Lawrence Henling at the Caltech X-ray crystallographic facility for assistance with XRD studies. The authors also thank Dr. Tzu-Pin Lin for editorial assistance and insightful suggestions. \n\nThis contribution is dedicated to the fond memory of Professor Gregory L. Hillhouse. \n\nThe authors declare no competing financial interest.", revision_no = "23", abstract = "An exciting challenge in transition metal catalyst design is to explore whether earth-abundant base metals such as Fe, Co, and Ni can mediate two-electron reductive transformations that their precious metal counterparts (e.g., Ru, Rh, Ir, and Pd) are better known to catalyze. Organometallic metalloboranes are an interesting design concept in this regard because they can serve as organometallic frustrated Lewis pairs. To build on prior studies with nickel metalloboranes featuring the DPB and ^(Ph)DPB^(Mes) ligands in the context of H_2 and silane activation and catalysis (DPB = bis(o-diisopropylphosphinophenyl)phenylborane, ^(Ph)DPB^(Mes) = bis(o-diphenylphosphinophenyl)mesitylborane), we now explore the reactivity of iron, [(DPB)Fe]_2(N_2), 1, and cobalt, (DPB)Co(N_2), 2, metalloboranes toward a series of substrates with E–H bonds (E = O, S, C, N) including phenol, thiophenol, benzo[h]quinoline, and 8-aminoquinoline. In addition to displaying high stoichiometric E–H bond activation reactivity, complexes 1 and 2 prove to be more active catalysts for the hydrosilylation of ketones and aldehydes with diphenylsilane relative to (^(Ph)DPB^(Mes))Ni. Indeed, 2 appears to be the most active homogeneous cobalt catalyst reported to date for the hydrosilylation of acetophenone under the conditions studied.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57986, title ="Evaluating Molecular Cobalt Complexes for the Conversion of N_2 to NH_3", author = "Del Castillo, Trevor J. and Thompson, Niklas B.", journal = "Inorganic Chemistry", volume = "54", number = "19", pages = "9256-9262", month = "October", year = "2015", doi = "10.1021/acs.inorgchem.5b00645", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150603-133712009", note = "© 2015 American Chemical Society. \n\nReceived: March 24, 2015. Publication Date (Web): May 22, 2015. \n\nSpecial Issue: Small Molecule Activation: From Biological Principles to Energy Applications. \n\nThis work was supported by the NIH (Grant GM 070757) and the Gordon and Betty Moore Foundation, and through the NSF via a GRFP award to T.J.D.C.", revision_no = "27", abstract = "We report a molecular Co−N_2 complex that generates a greater-than-stoichiometric yield of NH_3 (>200% NH_3 per Co−N_2 precursor) via the direct reduction of N_2 with protons and electrons. A comparison of the featured Co−N_2 complex with structurally related Co−N_2 and Fe−N_2 species shows how remarkably sensitive the N_2 reduction performance of potential precatalysts is. As discussed, structural and electronic effects are relevant to Co/Fe−N_2 conversion activity, including π basicity, charge state, and geometric flexibility.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/61003, title ="Preface for Small-Molecule Activation: From Biological Principles to Energy Applications. Part 2: Small Molecules Related to the Global Nitrogen Cycle", author = "Lehnert, Nicolai and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "54", number = "19", pages = "9229-9233", month = "October", year = "2015", doi = "10.1021/acs.inorgchem.5b02124", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20151012-134350094", note = "© 2015 American Chemical Society. \n\nPublished: October 5, 2015. \n\nN.L. thanks Profs. Gloria Coruzzi (New York University), Eric Hegg (Michigan State University), and Lance Seefeldt (Utah State University) for helpful discussions.", revision_no = "10", abstract = "The nitrogen cycle is among the most significant biogeochemical cycles on Earth because nitrogen is an essential nutrient for all forms of life. The largest contributor of nitrogen in the cycle is atmospheric dinitrogen (N_2), which is generally unavailable to plants by direct assimilation. Hence, access to fixed forms of nitrogen constitutes in many cases the most limiting factor for plant growth. This is in sharp contrast to carbon, which is easily taken up by plants from the atmosphere by fixing gaseous carbon dioxide (CO_2). The availability of nitrogen (and water) hence limits the ability to produce sufficient crops to feed our planet’s growing population.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/58802, title ="Characterization of an Fe≡N−NH_2 Intermediate Relevant to Catalytic N_2 Reduction to NH_3", author = "Anderson, John S. and Cutsail, George E., III", journal = "Journal of the American Chemical Society", volume = "137", number = "24", pages = "7803-7809", month = "June", year = "2015", doi = "10.1021/jacs.5b03432", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150708-084228936", note = "© 2015 American Chemical Society. Received: April 1, 2015.\nPublished: May 22, 2015. \n\nThis work was supported by the NIH (GM 070757 to J.C.P.,\nGM 111097 to B.M.H.), the NSF (MCB-1118613 to B.M.H.,\nDGE-0824162 to G.E.C.), an NSERC fellowship (to L.Z.), a\nCaltech Center for Environmental Microbial Interactions\nfellowship (to L.Z.), and the Gordon and Betty Moore\nFoundation. We thank the staff at Beamline 9−3, Stanford\nSynchrotron Radiation Lightsource (SSRL). SSRL is operated\nfor the DOE and supported by OBER and by the NIH, NIGMS\n(P41GM103393) and the NCRR (P31RR001209).", revision_no = "21", abstract = "The ability of certain transition metals to mediate the reduction of N_2 to NH_3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N_2 and mediate its protonation at one or more N atoms to furnish M(N_xH_y) species that can be characterized and, in turn, extrude NH_3. By contrast, the direct protonation of Fe–N_2 species to Fe(N_xH_y) products that can be characterized has been elusive. Herein, we show that addition of acid at low temperature to [(TPB)Fe(N_2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N–NH_2]^+, a doubly protonated hydrazido(2−) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N_2)][Na(12-crown-4)] can proceed along a distal or “Chatt-type” pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N–N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/58957, title ="Diiron Bridged-Thiolate Complexes That Bind N_2 at the Fe^(II)Fe^(II), Fe^(II)Fe^I, and Fe^IFe^I Redox States", author = "Creutz, Sidney E. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "137", number = "23", pages = "7310-7313", month = "June", year = "2015", doi = "10.1021/jacs.5b04738 ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150721-092526643", note = "© 2015 American Chemical Society.\n\nReceived: May 7, 2015; Published: June 3, 2015.\n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. We thank Larry Henling and Michael Takase for crystallographic assistance.", revision_no = "17", abstract = "All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N_2. Nonetheless, synthetic examples of transition metal model complexes that bind N_2 and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N_2-Fe(μ-SAr)Fe-N_2) across at least three oxidation states (Fe^(II)Fe^(II), Fe^(II)Fe^I, and Fe^IFe^I). The (N_2-Fe(μ-SAr)Fe-N_2) system undergoes reduction of the bound N_2 to produce NH_3 (∼50% yield) and can efficiently catalyze the disproportionation of N_2H_4 to NH_3 and N_2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/53565, title ="Hydricity of an Fe–H Species and Catalytic CO_2 Hydrogenation", author = "Fong, Henry and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "54", number = "11", pages = "5124-5135", month = "June", year = "2015", doi = "10.1021/ic502508p", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150112-104850035", note = "© 2014 American Chemical Society.\n\nReceived: October 18, 2014;\nPublished: December 31, 2014.\n\nThis material is based upon work performed by the Joint\nCenter of Artificial Photosynthesis, a DOE Energy Innovation\nHub, supported through the Office of Science of the U.S.\nDepartment of Energy under Award DE-SC0004993. The\nBercaw Group at the California Institute of Technology is\nacknowledged for their assistance with the high-pressure\nreactions. GC-FID instrumentation at the Environmental\nAnalysis Center (EAC) at the California Institute of\nTechnology was used in this work. We acknowledge Dr.\nNathan Dalleska of the EAC for his assistance with the GC-FID measurements.", revision_no = "23", abstract = "Despite renewed interest in carbon dioxide (CO_2) reduction chemistry, examples of homogeneous iron catalysts that hydrogenate CO_2 are limited compared to their noble-metal counterparts. Knowledge of the thermodynamic properties of iron hydride complexes, including M–H hydricities (ΔG_(H)–, could aid in the development of new iron-based catalysts. Here we present the experimentally determined hydricity of an iron hydride complex: (SiP^(iPr)_3)Fe(H_2)(H), Δ_(G)H– = 54.3 ± 0.9 kcal/mol [SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]−]. We also explore the CO_2 hydrogenation chemistry of a series of triphosphinoiron complexes, each with a distinct apical unit on the ligand chelate (Si–, C–, PhB–, N, B). The silyliron (SiP^(R)_3)Fe (R = iPr and Ph) and boratoiron (PhBP^(iPr)_3)Fe (PhBP^(iPr)_3 = [PhB(CH_2PiPr_2)_3]^−) systems, as well as the recently reported (CP^(iPr)_3)Fe (CP^(iPr)_3 = [C(o-C_6H_4PiPr_2)_3]^−), are also catalysts for CO_2 hydrogenation in methanol and in the presence of triethylamine, generating methylformate and triethylammonium formate at up to 200 TON using (SiP^(Ph)_3)FeCl as the precatalyst. Under stoichiometric conditions, the iron hydride complexes of this series react with CO_2 to give formate complexes. Finally, the proposed mechanism of the (SiP^(iPr)_3)-Fe system proceeds through a monohydride intermediate (SiPiPr3)Fe(H2)(H), in contrast to that of the known and highly active tetraphosphinoiron, (tetraphos)Fe (tetraphos = P(o-C_6H_4PPh_2)_3), CO_2 hydrogenation catalyst.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57184, title ="Reduction of CO_2 by Pyridine Monoimine Molybdenum Carbonyl Complexes: Cooperative Metal–Ligand Binding of CO_2", author = "Sieh, Daniel and Lacy, David C.", journal = "Chemistry: a European Journal", volume = "21", number = "23", pages = "8497-8503", month = "June", year = "2015", doi = "10.1002/chem.201500463", issn = "0947-6539", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150504-095804337", note = "© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.\n\nReceived: February 4, 2015. Article first published online: 29 Apr. 2015.\n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the US Department of Energy under Award Number DE-SC0004993. D.C.L. is supported by the National Institutes of Health (Award Number F32M106726). The authors thank Michael Takase and Lawrence M. Henling for help with the absorption correction of the X-ray data and David VanderVelde for helpful discussions concerning NMR spectroscopy. Jesse D. Froehlich, Charles W. Machan, and Matthew D. Sampson are thanked for the introduction to IR-SEC techniques.", revision_no = "24", abstract = "[(^(Ar)PMI)Mo(CO)_4] complexes (PMI=pyridine monoimine; Ar=Ph, 2,6-di-iso-propylphenyl) were synthesized and their electrochemical properties were probed with cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC). The complexes undergo a reduction at more positive potentials than the related [(bipyridine)Mo(CO)_4] complex, which is ligand based according to IR-SEC and DFT data. To probe the reaction product in more detail, stoichiometric chemical reduction and subsequent treatment with CO_2 resulted in the formation of a new product that is assigned as a ligand-bound carboxylate, [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)]^(2-), by NMR spectroscopic methods. The CO_2 adduct [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)]^(2-) could not be isolated and fully characterized. However, the C_C coupling between the CO_2\nmolecule and the PDI ligand was confirmed by X-ray crystallographic characterization of one of the decomposition products of [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)])^(2-).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/57773, title ="The Cobalt Hydride that Never Was: Revisiting Schrauzer’s “Hydridocobaloxime”", author = "Lacy, David C. and Roberts, Gerri M.", journal = "Journal of the American Chemical Society", volume = "137", number = "14", pages = "4860-4864", month = "April", year = "2015", doi = "10.1021/jacs.5b01838", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150522-100123322", note = "© 2015 American Chemical Society.\n\nReceived: February 18, 2015; published: March 23, 2015.\n\nThis work was supported by NSF Center for Chemical\nInnovation Solar Fuels Grant CHE-1305124. Larry Henling\nand Dr. Michael K. Takase are thanked for their help with X-ray crystallography. Sidney E. Creutz is thanked for assistance with SQUID measurements. D.C.L. would like to acknowledge the National Institute of Health (award number F32GM106726). G.M.R. would like to acknowledge the Peter A. Lindstrom, Jr. SURF fellowship.", revision_no = "14", abstract = "Molecular cobalt-dmg (dmg = dimethylglyoxime) complexes are an important class of electrocatalysts used heavily in mechanistic model studies of the hydrogen evolution reaction (HER). Schrauzer’s early isolation of a phosphine-stabilized “[H-Co^(III)(dmgH)_2P(nBu)_3]” complex has long provided circumstantial support for the plausible intermediacy of Co(III)-H species in HER by cobaloximes in solution. Our investigation of this complex has led to a reassignment of its structure as [Co^(II)(dmgH)_2P(nBu)_3], a complex that contains no hydride ligand and dimerizes to form an unsupported Co–Co bond in the solid state. A paramagnetic S = 3/2 impurity that forms during the synthesis of [Co^(II)(dmgH)_2P(nBu)_3] when exposed to adventitious oxygen has also been characterized. This impurity features a 1H NMR resonance at −5.06 ppm that was recently but erroneously attributed to the hydride resonance of “[H-Co^(III)(dmgH)_2P(nBu)_3]”. We draw attention to this reassignment because of its relevance to cobaloxime hydrides and HER catalysis and because Schrauzer’s “hydridocobaloxime” is often cited as the primary example of a bona fide hydride that can be isolated and characterized on this widely studied HER platform.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/56238, title ="Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices", author = "McCrory, Charles C. L. and Jung, Suho", journal = "Journal of the American Chemical Society", volume = "137", number = "13", pages = "4347-4357", month = "April", year = "2015", doi = "10.1021/ja510442p", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150331-092334778", note = "© 2015 American Chemical Society.\n\nReceived: October 10, 2014. Publication Date (Web): February 10, 2015.\n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. We thank Joel Haber for providing the NiFeCoCe-(a) and NiFeCoCe-(b) samples and Jesus M. Velazquez for his help in preparing the sputtered Ir and Ru-(b) samples. We also thank Slobodan Mitrovic, Natalie Becerra, and Fadl Saadi for their help with the collection of XPS data. In addition, we acknowledge many useful discussions with Nathan S. Lewis and Carl A. Koval.", revision_no = "23", abstract = "Objective comparisons of electrocatalyst activity and stability using standard methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, we use a standard protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochemically active surface area (ECSA) of 18 electrocatalysts for the hydrogen evolution reaction (HER) and 26 electrocatalysts for the oxygen evolution reaction (OER) under conditions relevant to an integrated solar water-splitting device in aqueous acidic or alkaline solution. Our primary figure of merit is the overpotential necessary to achieve a magnitude current density of 10 mA cm^(–2) per geometric area, the approximate current density expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm^(–2) with overpotentials <0.1 V in acidic and/or alkaline solutions. Among OER catalysts in acidic solution, no non-noble metal based materials showed promising activity and stability, whereas in alkaline solution many OER catalysts performed with similar activity achieving 10 mA cm–2 current densities at overpotentials of ∼0.33–0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alkaline solutions, while most HER catalysts showed much lower specific activity than Pt in both acidic and alkaline solutions. For select catalysts, additional secondary screening measurements were conducted including Faradaic efficiency and extended stability measurements.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/56788, title ="Visible Light Sensitized CO2 Activation by the Tetraaza [Co^(II)N_4H(MeCN)]^(2+) Complex Investigated by FT-IR Spectroscopy and DFT Calculations", author = "Zhang, M. and El-Roz, M.", journal = "Journal of Physical Chemistry C", volume = "119", number = "9", pages = "4645-4654", month = "March", year = "2015", doi = "10.1021/jp5127738", issn = "1932-7447", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150420-145653024", note = "© 2015 American Chemical Society. Received: December 22, 2014.\nRevised: February 11, 2015. Publication Date (Web): February 12, 2015.\n\nThis material is based on work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. M.E.-R. acknowledges the European Regional Development Fund Franco-British INTERREG IVA (Project E3C3, ref. 4274) for financial support, and Prof. Frederic Thibault-Starzyk for insightful discussions. D.C.L. would like to acknowledge the National Institute of Health (Award Number F32GM106726).", revision_no = "12", abstract = "In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO_2 catalyzed by the macrocyclic complex [Co^(II)N_4H]^(2+) (N_4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [Co^(II)N_4H]^(2+) resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)_3]^(2+) sensitizer yields an infrared band at 1670 cm^(–1) attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN_4H(MeCN)]^+ in CO_2 saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to Co^I in the first reduction, which is consistent with experimental d–d spectra of square Co(I) macrocycle complexes. The energy of the CO_2 adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN_4H(MeCN)]^+, which implies that only a fraction of the latter forms an adduct with CO_2. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [Co^I(N_4H) –•], and show that [Co^I(N_4H) –•] binds partially reduced CO_2 fairly strongly. Experimentally accessing [Co^I(N_4H) –•] with an Ir(bpy)_3 sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C^(18)O_2 also shows clear evidence for ^(18)O-substituted byproducts from CO_2 reduction to CO.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/56640, title ="Characterization of molecular cobalt complexes relevant to electrocatalytic CO_2 & H^+ reduction", author = "Lacy, David C. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "249", pages = "INOR-774", month = "March", year = "2015", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150414-131424017", note = "© 2015 American Chemical Society.", revision_no = "9", abstract = "Certain mol. cobalt complexes are competent in electrocatalytic transformations such as CO_2 and /or H+ redn. While\ninteresting from a purely chem. standpoint, these systems have also found interest because of their relevance to solar- energy\nrelated chem. transformations. The desire to lower the overpotentials necessary to achieve meaningful current densities has\nencouraged our investigations into the chem. and structural properties of the reduced compds. that are proposed to be involved\nin key mechanistic steps in hopes to gain insight into future catalyst design. In a typical proposed electrocatalytic\nmechanism, intermediates such as Co- CO_2 adducts or Co(III) - hydride complexes are invoked and are synthetic targets for our\nstudies. We have synthesized compds. with N_4 macrocyclic ligands that contain intra- mol. H- bond donors and /or phosphine\nligands in hopes to stabilize CO_2 adducts and hydride ligands bound to cobalt centers. Combined spectroscopic, X- ray\nstructural, and reactivity studies provide insights into the properties of these compds. and will be discussed herein. Specifically,\nwe have conducted a structure function study on an N_4 macrocyclic ligand with an N- H moiety that may be involved in\nstabilizing a CO_2 adduct that is subsequently reduced to form carbon monoxide in aq. acetonitrile. We have also prepd. another\nN_4 - macrocylcic cobalt complex that contains an axial phosphine ligand and have investigated putative hydride intermediates on\nthis platform.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52098, title ="Low-Temperature N_2 Binding to Two-Coordinate L_2Fe^0 Enables Reductive Trapping of L_2FeN_2^- and NH_3 Generation", author = "Ung, Gaël and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "54", number = "2", pages = "532-535", month = "January", year = "2015", doi = "10.1002/anie.201409454", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141124-110523536", note = "© 2014 Wiley-VCH Verlag GmbH & Co. Manuscript \n\nReceived: 24 Sep 2014. Article first published online: 13 Non 2014. Early View (Online Version of Record published before inclusion in an issue). \n\nThis work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. We thank Larry Henling and Michael K. Takase for crystallographic assistance.", revision_no = "19", abstract = "The two-coordinate [(CAAC)_2Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<−80\u2009°C). The resulting putative three-coordinate N_2 complex, [(CAAC)_2Fe(N_2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)_2FeN_2]^− at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes [(CAAC)_2Fe^I][BAr^F_4], [(CAAC)_2Fe^0], and [(CAAC)_2Fe^(−I)N_2]^− were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC_8 and HBAr^F_4^(⋅2)\u2009Et_2O at −95\u2009°C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N_2 reduction activity is highly temperature dependent, with significant N_2 reduction to NH_3 only occurring below −78\u2009°C. This reactivity profile tracks with the low temperatures needed for N_2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)_2FeN_2]^−.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/50072, title ="A 10^6‑Fold Enhancement in N_2‑Binding Affinity of an Fe_2(μ-H)_2 Core upon Reduction to a Mixed-Valence Fe^(II)Fe^I State", author = "Rittle, Jonathan and McCrory, Charles C. L.", journal = "Journal of the American Chemical Society", volume = "136", number = "39", pages = "13853-13862", month = "October", year = "2014", doi = "10.1021/ja507217v", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140926-134453593", note = "© 2014 American Chemical Society. ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. \n \nReceived: July 21, 2014. Publication Date (Web): September 3, 2014. \n\nThis work was supported by the N.I.H. (GM 070757) and the\nGordon and Betty Moore Foundation. J.R. was supported by an\nN.S.F. graduate research fellowship. We thank Larry Henling\nfor crystallographic assistance, Prof. Michael T. Green and\nElizabeth Onderko for assistance with the collection and\nanalysis of EXAFS data, Dr. Angelo di Bilio for assistance with EPR measurements, and Prof. George Rossman for assisting with near-IR spectrophotometry measurements.\n\n", revision_no = "27", abstract = "Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron–molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N_2 to NH_3 is unknown. One role of these ligands may be to facilitate N_2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N_2 depending on the redox state of the Fe_2(μ-H)_2 unit. An unusual example of a mixed-valent Fe^(II)(μ-H)^2Fe^I is described that displays a 10^6-fold enhancement of N_2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH_3 are produced upon exposure to proton and electron equivalents. The Fe(μ-H)Fe(N2_) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/50069, title ="Boryl−Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin\nHydrogenation", author = "Lin, Tzu-Pin and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "136", number = "39", pages = "13672-13683", month = "October", year = "2014", doi = "10.1021/ja504667f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140926-131812982", note = "© 2014 American Chemical Society. Received: May 9, 2014. Publication Date (Web): September 2, 2014. \n\nThis material is based upon work performed by the Joint\nCenter for Artificial Photosynthesis, a DOE Energy Innovation\nHub supported through the Office of Science of the U.S.\nDepartment of Energy under Award DE-SC0004993. We thank\nDr. David C. Leitch, Dr. Yichen Tan, and Dr. Charles C. McCrory for insightful discussions on kinetics. We also thank\nProf. Gerard Parkin for insightful discussions on bonding.", revision_no = "24", abstract = "New approaches toward the generation of late first-row metal catalysts that efficiently facilitate two-electron reductive transformations (e.g., hydrogenation) more typical of noble-metal catalysts is an important goal. Herein we describe the synthesis of a structurally unusual S = 1 bimetallic Co complex, [(^(Cy)PBP)CoH]_2 (1), supported by bis(phosphino)boryl and bis(phosphino)hydridoborane ligands. This complex reacts reversibly with a second equivalent of H_2 (1 atm) and serves as an olefin hydrogenation catalyst under mild conditions (room temperature, 1 atm H_2). A bimetallic Co species is invoked in the rate-determining step of the catalysis according to kinetic studies. A structurally related Ni^INi^I dimer, [(^(Ph)PBP)Ni]_2 (3), has also been prepared. Like Co catalyst 1, Ni complex 3 displays reversible reactivity toward H_2, affording the bimetallic complex [(^(Ph)PBHP)NiH]_2 (4). This reversible behavior is unprecedented for Ni^I species and is attributed to the presence of a boryl–Ni bond. Lastly, a series of monomeric (^(tBu)PBP)NiX complexes (X = Cl (5), OTf (6), H (7), OC(H)O (8)) have been prepared. The complex (^(tBu)PBP)NiH (7) shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans influence exerted by the boryl ligand.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52282, title ="Free H_2 Rotation vs Jahn−Teller Constraints in the Nonclassical Trigonal (TPB)Co−H_2 Complex", author = "Gunderson, William A. and Suess, Daniel L. M.", journal = "Journal of the American Chemical Society", volume = "136", number = "42", pages = "14998-15009", month = "September", year = "2014", doi = "10.1021/ja508117h", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141202-135435499", note = "© 2014 American Chemical Society. \n\nReceived: August 7, 2014. Published: September 22, 2014. Publication Date (Web): September 22, 2014. \n\nThis work is dedicated to Prof. Harden M. McConnell, whose\nrecognition of the role of quantum-statistical considerations in paramagnetic resonance spectroscopies occupies but a small place among his numerous scientific contributions, on the occasion of his 87th birthday. We thank Mrs. Junhong (Helen) He, Oak Ridge National Laboratory, for assistance with mounting crystal for neutron diffraction. This work was supported by the National Science Foundation (MCB 1118613, BMH; the NSF Center CHE-1305124, JCP; DGE-0824162, GEC). Research conducted at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy, under Contract No. DE-AC05-00OR22725 with UT Battelle, LLC.", revision_no = "15", abstract = "Proton exchange within the M–H_2 moiety of (TPB)Co(H_2) (Co–H_2; TPB = B(o-C_6H_4PiPr_2)_3) by 2-fold rotation about the M–H_2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^(iPr)_3)Fe(H_2) (Fe–H_2) (SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]). The g-values for Co–H_2 and Fe–H_2 show that both have the Jahn–Teller (JT)-active ^2E ground state (idealized C_3 symmetry) with doubly degenerate frontier orbitals, (e)^3 = [|m_L ± 2>]^3 = [x^2 – y^2, xy]^3, but with stronger linear vibronic coupling for Co–H_2. The observation of ^1H ENDOR signals from the Co–HD complex, ^2H signals from the Co–D_2/HD complexes, but no ^1H signals from the Co–H_2 complex establishes that H_2 undergoes proton exchange at 2 K through rotation around the Co–H_2 axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co–H_2 exhibits rotor-like behavior in solution because the underlying C_3 molecular symmetry combined with H_2 exchange creates a dominant 6-fold barrier to H_2 rotation. Fe–H_2 instead shows H_2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H_2(σ^*) π-backbonding that becomes dependent on the H_2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L_2–M–E axis (E = Si for Fe–H_2; E = B for Co–H_2). Notably, the isotropic ^1H/^2H hyperfine coupling to the diatomic of Co–H_2 is nearly 4-fold smaller than for Fe–H_2.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/55980, title ="Catalytic conversion of nitrogen to ammonia by a single-site Fe model complex", author = "Anderson, John S. and Rittle, Jonathan", journal = "Abstracts of Papers of the American Chemical Society", volume = "248", pages = "91-INOR", month = "August", year = "2014", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20150323-140621724", note = "© 2014 American Chemical Society.", revision_no = "11", abstract = "Nitrogen redn. to ammonia is one of the most fascinating chem. transformations mediated in biol. While it is widely\nappreciated that the iron- rich cofactors of nitrogenase enzymes facilitate this transformation, how they do so remains a\nmystery. Inorg. model complexes afford one approach to consider and test the viability of mechanistic proposals. Herein we\ndescribe a well- defined tris- phosphine borane iron catalyst that is able to catalyze the redn. of N to NH by the addn. of\nprotons and electrons at - 78 °C. A key design element of the featured catalyst is a hemi- labile ligand that may functionally\nmodel the interstitial light C- atom of the nitrogenase cofactor. The results described suggest that a single- iron- sitehypothesis\nfor biol. nitrogenfixation warrants further consideration.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/49617, title ="Two-Coordinate Fe^0 and Co^0 Complexes Supported by Cyclic (alkyl)(amino)carbenes", author = "Ung, Gaël and Rittle, Jonathan", journal = "Angewandte Chemie International Edition", volume = "53", number = "32", pages = "8427-8431", month = "August", year = "2014", doi = "10.1002/anie.201404078 ", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140911-135224989", note = "© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.\n\nReceived: April 7, 2014;\nPublished online: June 20, 2014.\n\nThis work was supported by the Gordon and Betty Moore\nFoundation (J.C.P.) and the DOE (DE-FG02-13ER16370) (G.B.). J.R.\nwas supported by a National Science Foundation graduate fellowship.\nWe thank Larry Henling and Michael K. Takase for crystallographic\nassistance.", revision_no = "14", abstract = "The CAAC [CAAC=cyclic (alkyl)(amino)carbene] family of carbene ligands have shown promise in stabilizing unusually low-coordination number transition-metal complexes in low formal oxidation states. Here we extend this narrative by demonstrating their utility in affording access to the first examples of two-coordinate formal Fe^0 and Co^0 [(CAAC)_2M] complexes, prepared by reduction of their corresponding two-coordinate cationic Fe^I and Co^I precursors. The stability of these species arises from the strong σ-donating and π-accepting properties of the supporting CAAC ligands, in addition to steric protection.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47147, title ="Oxygen nucleophiles as reaction partners in photoinduced, copper-catalyzed cross-couplings: O-arylations of phenols at room temperature", author = "Tan, Yichen and Muñoz-Molina, José María", journal = "Chemical Science", volume = "5", number = "7", pages = "2831-2835", month = "July", year = "2014", doi = "10.1039/c4sc00368c ", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140710-132652083", note = "© 2014 The Royal Society of Chemistry. \n\nReceived 3rd February 2014; Accepted 17th April 2014. First published online 22 Apr 2014. \n\nThis work was supported by the Gordon and Betty Moore Foundation (grant to J.C.P.) and the Council for International Exchange of Scholars (Fulbright Scholar award to J.M.M.-M.). We thank Daniel T. Ziegler for preliminary studies, Sidney E. Creutz for experimental assistance, and the reviewers for constructive comments.", revision_no = "11", abstract = "Most copper-catalyzed cross-couplings require an elevated reaction temperature. Recently, a photoinduced variant has been developed that enables C–X bond-forming reactions of certain nitrogen and sulfur nucleophiles to proceed under unusually mild conditions (−40 °C to room temperature). In view of the importance of carbon–oxygen bond construction in organic chemistry, the expansion of this photochemical approach to oxygen nucleophiles is an important objective. In this report, we establish that, in the presence of light and an inexpensive copper pre-catalyst (CuI), a wide array of phenols and aryl iodides can be coupled to generate diaryl ethers under mild conditions (room temperature) in the presence of a variety of functional groups. Our studies indicate that a Cu(I)–phenoxide complex is a viable intermediate in photoinduced C–O bond-formation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/45241, title ="Low-Spin Pseudotetrahedral Iron(I) Sites in Fe_2(μ-S) Complexes", author = "Anderson, John S. and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "53", number = "23", pages = "5978-5981", month = "June", year = "2014", doi = "10.1002/anie.201401018", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140428-102317878", note = "© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.\nArticle first published online: 17 Apr. 2014. Manuscript Received: 30 Jan 2014.\n\nThis research was supported by the NIH (GM 070757). J.S.A.\ngratefully acknowledges a GRFP award from the NSF. Po-Heng Lin,\nJay Winkler, Bruce Brunschwig, Harry Gray, and Dave Harris are\nthanked for helpful discussions.", revision_no = "24", abstract = "Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L_3Fe)_2(μ-S)] complexes that were isolated and characterized in the low-valent oxidation states Fe^(II) S Fe^(II), Fe^(II) S Fe^I, and Fe^I S Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe_2(μ-S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low-spin S=1/2 states upon reduction from Fe^(II) to Fe^I. The possibility of accessing low-spin, pseudotetrahedral Fe^I sites compatible with S^(2−) as a ligand was previously unknown.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/45504, title ="Studies of Cobalt-Mediated Electrocatalytic CO_2 Reduction Using a Redox-Active Ligand", author = "Lacy, David C. and McCrory, Charles C. L.", journal = "Inorganic Chemistry", volume = "53", number = "10", pages = "4980-4988", month = "May", year = "2014", doi = "10.1021/ic403122j", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140505-142420147", note = "© 2014 American Chemical Society. ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. \n\nReceived: December 30, 2013; Published: April 28, 2014. \n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation\nHub, supported through the Office of Science of the U.S.\nDepartment of Energy under Award Number DE-SC0004993.\nD.C.L. would also like to acknowledge the National Institutes\nof Health (Award Number F32GM106726). The authors\nwould also like to thank Tzu-Pin Lin, Michael Takase, and\nLawrence Henling for help with crystallography, and Kyle\nCummins and Slobodan Mitrovic for help with XPS. Clifford\nKubiak is also thanked for many insightful discussions.", revision_no = "22", abstract = "The cobalt complex [Co^(III)N_4H(Br)_2]+ (N_4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(7),2,11,13,15-pentaene) was used for electrocatalytic CO_2 reduction in wet MeCN with a glassy carbon working electrode. When water was employed as the proton source (10 M in MeCN), CO was produced (f_(CO)= 45% ± 6.4) near the Co^(I/0) redox couple for [Co^(III)N_4H(Br)_2]+ (E_(1/2) = −1.88 V FeCp_2^(+/0)) with simultaneous H_2 evolution (f_(H2)= 30% ± 7.8). Moreover, we successfully demonstrated that the catalytically active species is homogeneous through the use of control experiments and XPS studies of the working glassy-carbon electrodes. As determined by cyclic voltammetry, CO_2 catalysis occurred near the formal CoI/0redox couple, and attempts were made to isolate the triply reduced compound (“[Co^0N_4H]”). Instead, the doubly reduced (“Co^I”) compounds [CoN4] and [CoN_4H(MeCN)]+ were isolated and characterized by X-ray crystallography. Their molecular structures prompted DFT studies to illuminate details regarding their electronic structure. The results indicate that reducing equivalents are stored on the ligand, implicating redox noninnocence in the ligands for H_2 evolution and CO_2 reduction electrocatalysis.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52538, title ="Phosphido-diphosphine pincer group 3 complexes as efficient initiators for lactide polymerization", author = "Mazzeo, Mina and Lamberti, Marina", journal = "Journal of Polymer Science Part A: Polymer Chemistry", volume = "48", number = "6", pages = "1374-1382", month = "March", year = "2014", doi = "10.1002/pola.23899", issn = "0887-624X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-094157817", note = "© 2010 Wiley Periodicals, Inc.\n\nReceived 30 September 2009; accepted 16 December 2009.\n\nThe authors thank Dr. Mariagrazia Napoli for GPC analyses, Dr.\nAssunta Napolitano and Dr. Daniela Eletto for MALDI-TOF-MS\nanalysis, and Patrizia Iannece for elemental analyses. M. Mazzeo thanks Prof. Moshe Kol for engaging discussions.", revision_no = "12", abstract = "New Group 3 metal complexes of the type [LM(CH_2SiMe_3)_2(THF)n] supported by tridentate phosphido-diphosphine ligands [(o-C_6H_4PR_2)_2PH; L1-H: R = iPr; L2-H: R = Ph] have been synthesized by reaction of L1-H and L2-H with [M(CH_2SiMe_3)_3(THF)2)] (M = Y and Sc). All the new complexes [(o-C_6H_4PR_2)_2PM(CH_2SiMe_3)_2(THF)n] [M = Y, R = iPr (1), R = Ph (2); M = Sc, R = iPr (3), R = Ph (4)] were studied as initiators for the ring opening polymerization of lactide. The yttrium complexes (1 and 2) exhibited high activity and good polymerization control, shown by the linear fits in the plot of number-averaged molecular weight (Mn) versus the percentage conversion and versus the monomer/initiator ratio and by the low polydispersity index values. Interestingly, very good molar-mass control was observed even when L-Lactide was polymerized in the absence of solvent at 130 °C. A good molar-mass control but lower activities were observed in the polymerization reaction of lactide promoted by the analogous scandium complexes 3 and 4.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43521, title ="Photoinduced, Copper-Catalyzed Alkylation of Amides with Unactivated Secondary Alkyl Halides at Room Temperature", author = "Do, Hien-Quang and Bachman, Shoshana", journal = "Journal of the American Chemical Society", volume = "136", number = "5", pages = "2162-2167", month = "February", year = "2014", doi = "10.1021/ja4126609", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140127-095430890", note = "© 2014 American Chemical Society. \n\nReceived: December 12, 2013. Publication Date (Web): January 21, 2014. \n\nThis work was supported by the Gordon and Betty Moore Foundation. We acknowledge Prof. Rick L. Danheiser (MIT) for helpful discussions, and we thank Trixia M. Buscagan, Sidney E. Creutz, Gregory P. Harlow, Dr. Nathan D. Schley, Dr. David VanderVelde (Caltech NMR Facility), and Dr. Scott C. Virgil (Caltech Center for Catalysis and Chemical Synthesis, supported by the Gordon and Betty Moore Foundation) for experimental assistance.", revision_no = "19", abstract = "The development of a mild and general method for the alkylation of amides with relatively unreactive alkyl halides (i.e., poor substrates for S_N2 reactions) is an ongoing challenge in organic synthesis. We describe herein a versatile transition-metal-catalyzed approach: in particular, a photoinduced, copper-catalyzed monoalkylation of primary amides. A broad array of alkyl and aryl amides (as well as a lactam and a 2-oxazolidinone) couple with unactivated secondary (and hindered primary) alkyl bromides and iodides using a single set of comparatively simple and mild conditions: inexpensive CuI as the catalyst, no separate added ligand, and C–N bond formation at room temperature. The method is compatible with a variety of functional groups, such as an olefin, a carbamate, a thiophene, and a pyridine, and it has been applied to the synthesis of an opioid receptor antagonist. A range of mechanistic observations, including reactivity and stereochemical studies, are consistent with a coupling pathway that includes photoexcitation of a copper–amidate complex, followed by electron transfer to form an alkyl radical.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43344, title ="Catalytic Reduction of N_2 to NH_3 by an Fe−N_2 Complex Featuring a C‑Atom Anchor", author = "Creutz, Sidney E. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "136", number = "3", pages = "1105-1115", month = "January", year = "2014", doi = "10.1021/ja4114962", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140113-132603289", note = "© 2013 American Chemical Society. \n\nPublished In Issue January 22, 2014; Article ASAP January 09, 2014; Just Accepted ManuscriptDecember 18, 2013; Received: November 11, 2013.\n\nThis work was supported by the NIH (GM 070757) and the\nGordon and Betty Moore Foundation, and through the NSF via\na GRFP award to S.E.C. Larry Henling and Dr. Tzu-Pin Lin are\nthanked for their assistance with X-ray crystallography. Jon\nRittle is thanked for testing the catalytic activity of [C^(Si)P^(Ph)_3]-FeN_2^−. ", revision_no = "19", abstract = "While recent spectroscopic studies have established the presence of an interstitial carbon atom at the center of the iron–molybdenum cofactor (FeMoco) of MoFe-nitrogenase, its role is unknown. We have pursued Fe–N_2 model chemistry to explore a hypothesis whereby this C-atom (previously denoted as a light X-atom) may provide a flexible trans interaction with an Fe center to expose an Fe–N_2 binding site. In this context, we now report on Fe complexes of a new tris(phosphino)alkyl (CP^(iPr)_3) ligand featuring an axial carbon donor. It is established that the iron center in this scaffold binds dinitrogen trans to the C_(alkyl)-atom anchor in three distinct and structurally characterized oxidation states. Fe–C_(alkyl) lengthening is observed upon reduction, reflective of significant ionic character in the Fe–C_(alkyl) interaction. The anionic (CP^(iPr)_3)FeN_2^– species can be functionalized by a silyl electrophile to generate (CP^(iPr)_3)Fe–N_2SiR_3. (CP^(iPr)_3)FeN_2^– also functions as a modest catalyst for the reduction of N_2 to NH_3 when supplied with electrons and protons at −78 °C under 1 atm N_2 (4.6 equiv NH_3/Fe).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43824, title ="A d^10 Ni–(H_2) Adduct as an Intermediate in H-H Oxidative Addition across a Ni-B Bond", author = "Harman, W. Hill and Lin, Tzu-Pin", journal = "Angewandte Chemie International Edition", volume = "53", number = "4", pages = "1081-1086", month = "January", year = "2014", doi = "10.1002/anie.201308175", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140213-154635896", note = "© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.\n\nPublished online: December 9, 2013; Received: September 17, 2013.\n\nSupporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201308175.\n\nThis research was supported by the NSF Center for Chemical Innovation: Solar Fuels (grant CHE-0802907) and by the Gordon and Betty Moore Foundation. We thank Prof. Christopher C. Cummins and Dr. Smith Nielsen for insightful discussions.", revision_no = "15", abstract = "Bifunctional E-H activation offers a promising approach for the design of two-electron-reduction catalysts with late first-row metals, such as Ni. To this end, we have been pursuing H_2 activation reactions at late-metal boratranes and herein describe a diphosphine–borane-supported Ni—(H_2) complex, [(^(Ph)DPB^(iPr))Ni(H_2)], which has been characterized in solution. ^1H NMR spectroscopy confirms the presence of an intact H_2 ligand. A range of data, including electronic-structure calculations, suggests a d^(10) configuration for [(^(Ph)DPB^(iPr))Ni(H_2)] as most appropriate. Such a configuration is highly unusual among transition-metal H_2 adducts. The nonclassical H_2 adduct is an intermediate in the complete activation of H_2 across the Ni-B interaction. Reaction-coordinate analysis suggests synergistic activation of the H_2 ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d^(10) Ni—(H_2) interaction and in the H—H cleavage step. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43006, title ="Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction", author = "McCrory, Charles C. L. and Jung, Suho", journal = "Journal of the American Chemical Society", volume = "135", number = "45", pages = "16977-16987", month = "November", year = "2013", doi = "10.1021/ja407115p", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131213-151603830", note = "© 2013 American Chemical Society. \n\nReceived: July 11, 2013. Publication Date (Web): October 30, 2013. \n\nThis material is based upon work performed by the Joint\nCenter for Artificial Photosynthesis, a DOE Energy Innovation\nHub, supported through the Office of Science of the U.S.\nDepartment of Energy under award no. DE-SC0004993. We\nare grateful for the many useful insights we have received\nregarding this work from various members of the electrochemistry community. In particular, we would like to\nacknowledge useful discussions with Allen J. Bard, Fred C.\nAnson, Nathan S. Lewis, Carl A. Koval, Manuel P. Soriaga, and\nHans-Joachim Lewerenz. XPS data was collected at the\nMolecular Materials Research Center of the Beckman Institute\nof the California Institute of Technology. The authors declare no competing financial interest.", revision_no = "12", abstract = "Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm^(–2) per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoO_x, CoPi, CoFeO_x, NiO_x, NiCeO_x, NiCoO_x, NiCuO_x, NiFeO_x, and NiLaO_x. The oxygen-evolving activity of an electrodeposited IrO_x catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alkaline solution, every non-noble metal system achieved 10 mA cm^(–2) current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrO_x was unstable under oxidative conditions in acidic solutions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43544, title ="Facile Si–H bond activation and hydrosilylation catalysis mediated by a nickel–borane complex", author = "MacMillan, Samantha N. and Harman, W. Hill", journal = "Chemical Science", volume = "5", number = "2", pages = "590-597", month = "October", year = "2013", doi = "10.1039/c3sc52626g", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140129-104856793", note = "© 2014 Royal Society of Chemistry.\n\nReceived 18th September 2013; accepted 22nd October 2013.\n\nFirst published online 22 Oct 2013.\nThis work was supported by the National Science Foundation\nCenter for Chemical Innovation on Solar Fuels (CCI Solar, Grant CHE-1305124) and the Gordon and Betty Moore Foundation.\nWe thank David VanderVelde for assistance with NMR\nexperiments.", revision_no = "12", abstract = "Metal–borane complexes are emerging as promising systems for study in the context of bifunctional catalysis. Herein we describe diphosphineborane nickel complexes that activate Si–H bonds and catalyze the hydrosilylation of aldehydes. Treatment of [^(Mes)DPB^(Ph)]Ni (1) ([^(Mes)DPB^Ph] = ^(Mes)B(o-Ph_2PC_6H_4)_2) with organosilanes affords the complexes [^(Mes)DPB^(Ph)](μ-H)NiE (E = SiH_2Ph (3), SiHPh_2 (4)). Complex 4 is in solution equilibrium with 1 and the thermodynamic and kinetic parameters of their exchange have been characterized by NMR spectroscopy. Complex 1 is a catalyst for the hydrosilylation of a range of para-substituted benzaldehydes. Mechanistic studies on this reaction via multinuclear NMR spectroscopy are consistent with the intermediacy of a borohydrido-Ni-siloxyalkyl species.\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42256, title ="Boryl-Mediated Reversible H_2 Activation at Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer Hydrogenation", author = "Lin, Tzu-Pin and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "135", number = "41", pages = "15310-15313", month = "October", year = "2013", doi = "10.1021/ja408397v", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131105-144406165", note = "© 2013 American Chemical Society. \n\nPublication Date (Web): September 30, 2013. Received: August 13, 2013. \n\nThis material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. \n\nThe authors declare no competing financial interest.", revision_no = "15", abstract = "We describe the synthesis of a cobalt(I)–N2 complex (2) supported by a meridional bis-phosphino-boryl (PBP) ligand. Complex 2 undergoes a clean reaction with 2 equiv of dihydrogen to afford a dihydridoboratocobalt dihydride (3). The ability of boron to switch between a boryl and a dihydridoborate conformation makes possible the reversible conversion of 2 and 3. Complex 3 reacts with HMe_2N–BH_3 to give a hydridoborane cobalt tetrahydridoborate complex. We explore this boryl–cobalt system in the context of catalytic olefin hydrogenation as well as amine–borane dehydrogenation/transfer hydrogenation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/42045, title ="Fe–N_2/CO complexes that model a possible role for the interstitial C atom of FeMo-cofactor (FeMoco)", author = "Rittle, Jonathan and Peters, Jonas C.", journal = "Proceedings of the National Academy of Sciences of the United States of America", volume = "110", number = "40", pages = "15898-15903", month = "October", year = "2013", doi = "10.1073/pnas.1310153110 ", issn = "0027-8424", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131024-101957270", note = "© 2013 National Academy of Sciences.\n\nEdited by Douglas C. Rees, Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA, and approved August 6, 2013 (received for review May 29, 2013).\n\nWe thank Larry Henling and Charlene Tsay for\ncrystallographic assistance as well as Dr. Angelo di Bilio for assistance with EPR measurements. This work was supported by the National Institutes of Health (GM 070757) and the Gordon and Betty Moore Foundation. J.R. was supported by a National Science Foundation graduate fellowship.\n\nAuthor contributions: J.R. and J.C.P. designed research; J.R. performed research; J.R.\nand J.C.P. analyzed data; and J.R. and J.C.P. wrote the paper.\nThe authors declare no conflict of interest.\nThis article is a PNAS Direct Submission.\nData deposition: The atomic coordinates have been deposited in the Cambridge Structural\nDatabase, Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United\nKingdom (CSD reference nos. 909104–909108, 941163, and 941164).", revision_no = "16", abstract = "We report here a series of four- and five-coordinate Fe model complexes that feature an axial tri(silyl)methyl ligand positioned trans to a substrate-binding site. This arrangement is used to crudely model a single-belt Fe site of the FeMo-cofactor that might bind N_2 at a position trans to the interstitial C atom. Reduction of a trigonal pyramidal Fe(I) complex leads to uptake of N_2 and subsequent functionalization furnishes an open-shell Fe–diazenido complex. A related series of five-coordinate Fe–CO complexes stable across three redox states is also described. Spectroscopic, crystallographic, and Density Functional Theory (DFT) studies of these complexes suggest that a decrease in the covalency of the Fe–C_alkyl interaction occurs upon reduction and substrate binding. This leads to unusually long Fe–C_alkyl bond distances that reflect an ionic Fe–C bond. The data presented are contextualized in support of a hypothesis wherein modulation of a belt Fe–C interaction in the FeMo-cofactor facilitates substrate binding and reduction. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41645, title ="Catalytic conversion of nitrogen to ammonia by an iron model complex", author = "Anderson, John S. and Rittle, Jonathan", journal = "Nature", volume = "501", number = "7465", pages = "84-87", month = "September", year = "2013", doi = "10.1038/nature12435 ", issn = "0028-0836", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131002-154839797", note = "© 2013 Macmillan Publishers Limited.\n\nReceived 08 February 2013; Accepted 04 July 2013; Published online 04 September 2013.\n\nThis work was supported by the NIH (GM 070757) and the\nGordon and Betty Moore Foundation. A. Takaoka is thanked for developing the calibration curves used for ammonia and hydrazine quantification. D. Rees and D. Newman are acknowledged for many discussions.\n\nAuthor Contributions: J.S.A., J.R. and J.C.P. designed the study. J.S.A. and J.R.\nconducted the experiments. J.S.A., J.R. and J.C.P. interpreted the data. J.S.A., J.R. and\nJ.C.P. wrote the manuscript.", revision_no = "23", abstract = "The reduction of nitrogen (N_2) to ammonia (NH_3) is a requisite transformation for life. Although it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation, how they do so remains poorly understood. A central element of debate has been the exact site or sites of N_2 coordination and reduction. In synthetic inorganic chemistry, an early emphasis was placed on molybdenum because it was thought to be an essential element of nitrogenases and because it had been established that well-defined molybdenum model complexes could mediate the stoichiometric conversion of N_2 to NH_3 (ref. 9). This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature molybdenum centres. However, it is now thought that iron is the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data have implicated iron instead of molybdenum as the site of N_2 binding in the FeMo-cofactor. Here we describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of N_2 to NH_3 under mild conditions, and in which more than 40 per cent of the proton and reducing equivalents are delivered to N_2. Our results indicate that a single iron site may be capable of stabilizing the various N_xH_y intermediates generated during catalytic NH_3 formation. Geometric tunability at iron imparted by a flexible iron–boron interaction in our model system seems to be important for efficient catalysis. We propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role, perhaps by enabling a single iron site to mediate the enzymatic catalysis through a flexible iron–carbon interaction.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/43954, title ="A Versatile Approach to Ullmann C−N Couplings at Room Temperature: New Families of Nucleophiles and Electrophiles for Photoinduced, Copper-Catalyzed Processes", author = "Ziegler, Daniel T. and Choi, Junwon", journal = "Journal of the American Chemical Society", volume = "135", number = "35", pages = "13107-13112", month = "September", year = "2013", doi = "10.1021/ja4060806", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140224-141339918", note = "© 2013 American Chemical Society. \n\nPublished In Issue September 04, 2013; Article ASAP August 22, 2013; Received: June 17, 2013. \n\nJ.C. and J.M.M.-M. contributed equally to this work. \n\nThis work was supported by the Gordon and Betty Moore Foundation (grant to J.C.P.), the Kwanjeong Educational Foundation (fellowship to J.C.), and the Council for International Exchange of Scholars (Fulbright Scholar award to J.M.M.-M.). We thank the Caltech Center for Catalysis and Chemical Synthesis for use of instrumentation. D.T.Z. and J.C. are graduate students in the Department of Chemistry at the Massachusetts Institute of Technology. \n\nExperimental procedures and compound characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.", revision_no = "17", abstract = "The use of light to facilitate copper-catalyzed cross-couplings of nitrogen nucleophiles can enable C−N bond formation to occur under unusually mild conditions. In this study, we substantially expand the scope of such processes, establishing that this approach is not limited to reactions of carbazoles with iodobenzene and alkyl halides. Specifically, we demonstrate for the first time that other nitrogen nucleophiles (e.g., common pharmacophores such as indoles, benzimidazoles, and imidazoles) as well\nas other electrophiles (e.g., hindered/deactivated/heterocyclic aryl iodides, an aryl bromide, an activated aryl chloride, alkenyl halides, and an alkynyl bromide) serve as suitable partners. Photoinduced C−N bond formation can be achieved at room temperature using a common procedure with an inexpensive catalyst (CuI) that does\nnot require a ligand coadditive and is tolerant of moisture and a variety of functional groups.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41610, title ="A CO-Derived Iron Dicarbyne That Releases Olefin upon Hydrogenation", author = "Suess, Daniel L. M. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "135", number = "34", pages = "12580-12583", month = "August", year = "2013", doi = "10.1021/ja406874k ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131002-111755379", note = "© 2013 American Chemical Society.\n\nReceived: July 5, 2013;\nPublished: August 9, 2013.\n\nWe acknowledge the NIH (GM070757) and the Beckman\nInstitute for funding and thank Lawrence Henling for assistance\nwith XRD studies.", revision_no = "15", abstract = "An iron diphosphineborane platform that was previously reported to facilitate a high degree of N_2 functionalization is herein shown to effect reductive CO coupling. Disilylation of an iron dicarbonyl precursor furnishes a structurally unprecedented iron dicarbyne complex. Several complexes related to this process are also characterized which allows for a comparative analysis of their respective Fe–B and Fe–C bonding. Facile hydrogenation of the iron dicarbyne at ambient temperature and 1 atm H_2 results in release of a CO-derived olefin.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41360, title ="Selective Nitrite Reduction at Heterobimetallic CoMg Complexes", author = "Uyeda, Christopher and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "135", number = "32", pages = "12023-12031", month = "August", year = "2013", doi = "10.1021/ja4053653 ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130917-100711754", note = "© 2013 American Chemical Society. \n\nReceived: May 29, 2013; Published: July 18, 2013. \n\nThis work was supported by the National Science Foundation Center for Chemical Innovation on Solar Fuels (CCI Solar, Grant CHE-0802907) and the Gordon and Betty Moore Foundation. We thank Larry M. Henling for assistance with crystallography, Angelo Di Bilio for assistance with EPR measurements, and David Lacy for assistance with IR spectroelectrochemical experiments.", revision_no = "13", abstract = "Heme-containing nitrite reductases bind and activate nitrite by a mechanism that is proposed to involve interactions with Brønsted acidic residues in the secondary coordination sphere. To model this functionality using synthetic platforms that incorporate a Lewis acidic site, heterobimetallic CoMg complexes supported by diimine–dioxime ligands are described. The neutral (μ-NO_2)CoMg species 3 is synthesized from the [(μ-OAc)(Br)CoMg]+ complex 1 by a sequence of one-electron reduction and ligand substitution reactions. Data are presented for a redox series of nitrite adducts, featuring a conserved μ-(η^1-N:η^1-O)-NO_2 motif, derived from this synthon. Conditions are identified for the proton-induced N–O bond heterolysis of bound NO_2– in the most reduced member of this series, affording the [(NO)(Cl)CoMg(H_2O)]+ complex 6. Reduction of this complex followed by protonation leads to the evolution of free N_2O. On the basis of these stoichiometric reactivity studies, the competence of complex 1 as a NO_2– reduction catalyst is evaluated using electrochemical methods. In bulk electrolysis experiments, conducted at −1.2 V vs SCE using Et_3NHCl as a proton source, N_2O is produced selectively without the competing formation of NH_3, NH_2OH, or H_2.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41649, title ="Pacman and Hangman Metal Tetraazamacrocycles", author = "Lee, Chang Hoon and Villágran, Dino", journal = "ChemSusChem", volume = "6", number = "8", pages = "1541-1544", month = "August", year = "2013", issn = "1864-5631", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131003-085853977", note = "© 2013 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.\n\nReceived: January 21, 2013;\nRevised: April 11, 2013;\nPublished online on July 26, 2013.\n\nThis research was supported by DOE grant DE-FG02-05ER15745\n(D.G.N.) and the NSF Center for Chemical Innovation: Powering\nthe Planet CHE-0802907 (J.C.P.). We thank Prof. M. Dinča for SEM\nand EDX analysis.", revision_no = "15", abstract = "Metal complexes of derivatized 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentane (bapa) ligands were prepared from 4-substituted diacetylpyridine derivatives by templated condensation with 3,3′-diaminodipropylamine in the presence of a metal halide or nitrate. The diacetylpyridine derivatives with Pacman and Hangman scaffolds are delivered from borylation of the 4-postion of diacetylpyridine and subsequent Suzuki coupling with the appropriate Hangman or Pacman backbone. Electrochemical examination of the parent [Co(bapa)]^(2+) scaffold reveals it to be a catalyst for the hydrogen evolution reaction (HER) in acetonitrile. Similar studies of the Hangman complex appear to be obscured by trace amounts of residual palladium remaining from the Suzuki coupling reaction to provide a cautionary note for the use of such cross-coupling methodologies in the preparation of HER catalysts.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/41700, title ="CO_2 reduction by Fe(I): solvent control of C-O cleavage versus C-C coupling", author = "Saouma, Caroline T. and Lu, Connie C.", journal = "Chemical Science", volume = "4", number = "10", pages = "4042-4051", month = "July", year = "2013", doi = "10.1039/c3sc51262b ", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20131007-093329618", note = "© 2013 The Royal Society of Chemistry.\n\nReceived 07 May 2013, Accepted 11 Jul 2013,\nFirst published online 12 Jul 2013.\n\nThis work was generously supported by the NSF (CHE-0750234)\nand the Gordon and Betty Moore Foundation. C. T. S. is grateful\nfor an NSF graduate fellowship. We are also grateful to the\nMassachusetts Institute of Technology, Cambridge, Massachusetts,\n02139, where some of the research described was\nconducted.", revision_no = "13", abstract = "This manuscript explores the product distribution of the reaction of carbon dioxide with reactive iron(I)\ncomplexes supported by tris(phosphino)borate ligands, [PhBP^R_3]- ([PhBP^R_3]- =[PhB(CH_2PR_2)_3]-; R = CH_2Cy,Ph, ^iPr, mter; mter = 3,5-meta-terphenyl). Our studies reveal an interesting and unexpected role for the\nsolvent medium with respect to the course of the CO_2 activation reaction. For instance, exposure of\nmethylcyclohexane (MeCy) solutions of [PhBP^(CH_2Cy)_3 ]Fe(PR’_3) to CO_2 yields the partial decarbonylation product\n{[PhBP^(CH_2Cy)_3 ]Fe}_2(µ-O)(µ-CO). When the reaction is instead carried out in benzene or THF, reductive coupling of CO_2 occurs to give the bridging oxalate species {[PhBP^(CH_2Cy_3 ]Fe}_2(µ- κOO’: κOO’-oxalato).\nReaction studies aimed at understanding this solvent effect are presented, and suggest that the product\nprofile is ultimately determined by the ability of the solvent to coordinate the iron center. When more\nsterically encumbering auxiliary ligands are employed to support the iron(I) center (i.e., [PhBP^(Ph)_3]- and [PhBP^(iPr)_3 ]-), complete decarbonylation is observed to afford structurally unusual diiron(II) products of the\ntype {[PhBP^R_3]Fe}_2(µ-O). A mechanistic hypothesis that is consistent with the collection of results described is offered, and suggests that reductive coupling of CO_2 likely occurs from an electronically saturated\n“Fe^(II)–CO_2-” species.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39911, title ="A New Family of Nucleophiles for Photoinduced, Copper-Catalyzed Cross-Couplings via Single-Electron Transfer: Reactions of Thiols with Aryl Halides Under Mild Conditions (0 °C)", author = "Uyeda, Christopher and Tan, Yichen", journal = "Journal of the American Chemical Society", volume = "135", number = "25", pages = "9548-9552", month = "June", year = "2013", doi = " 10.1021/ja404050f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130814-104008585", note = "© 2013 American Chemical Society. \n\nReceived: April 23, 2013; Published: May 23, 2013. \n\nThis work was supported in part by the Gordon and Betty Moore Foundation. Insightful discussions with Kenneth J. Lotito as well as assistance with X-ray crystallography from Larry M. Henling are gratefully acknowledged.", revision_no = "16", abstract = "Building on the known photophysical properties of well-defined copper–carbazolide complexes, we have recently described photoinduced, copper-catalyzed N-arylations and N-alkylations of carbazoles. Until now, there have been no examples of the use of other families of heteroatom nucleophiles in such photoinduced processes. Herein, we report a versatile photoinduced, copper-catalyzed method for coupling aryl thiols with aryl halides, wherein a single set of reaction conditions, using inexpensive CuI as a precatalyst without the need for an added ligand, is effective for a wide range of coupling partners. As far as we are aware, copper-catalyzed C–S cross-couplings at 0 °C have not previously been achieved, which renders our observation of efficient reaction of an unactivated aryl iodide at −40 °C especially striking. Mechanistic investigations are consistent with these photoinduced C–S cross-couplings following a SET/radical pathway for C–X bond cleavage (via a Cu(I)–thiolate), which contrasts with nonphotoinduced, copper-catalyzed processes wherein a concerted mechanism is believed to occur.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39327, title ="Heterolytic H_2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane", author = "Fong, Henry and Moret, Marc-Etienne", journal = "Organometallics", volume = "32", number = "10", pages = "3053-3062", month = "May", year = "2013", doi = "10.1021/om400281v", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130711-154244924", note = "© 2013 American Chemical Society. \n\nReceived: April 3, 2013; published: May 6, 2013. \n\nWe acknowledge the Gordon and Betty Moore Foundation and\nthe NIH (GM070757) for funding. M.-E.M. acknowledges a\nFellowship for Advanced Researchers from the Swiss National\nScience Foundation. Dr. Charlene Tsay and Lawrence Henling\nare acknowledged for assistance with X-ray crystallography, and Dr. David VanderVelde is acknowledged for assistance with NMR spectroscopy. We acknowledge the Gordon and Betty Moore Foundation, the Beckman Institute, and the SanofiAventis BRP at Caltech for their generous support of the Molecular Observatory.", revision_no = "21", abstract = "Reversible, heterolytic addition of H_2 across an iron–boron bond in a ferraboratrane with formal hydride transfer to the boron gives iron-borohydrido-hydride complexes. These compounds catalyze the hydrogenation of alkenes and alkynes to the respective alkanes. Notably, the boron is capable of acting as a shuttle for hydride transfer to substrates. The results are interesting in the context of heterolytic substrate addition across metal–boron bonds in metallaboratranes and related systems, as well as metal–ligand bifunctional catalysis.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/39006, title ="Transition-Metal-Catalyzed Alkylations of Amines with Alkyl Halides: Photoinduced, Copper-Catalyzed Couplings of Carbazoles", author = "Bissember, Alex C. and Lundgren, Rylan J.", journal = "Angewandte Chemie International Edition", volume = "52", number = "19", pages = "5129-5133", month = "May", year = "2013", doi = "10.1002/anie.201301202 ", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130620-145951306", note = "© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. \n\nReceived: February 11, 2013; Published online: April 8, 2013. \n\nThis work was supported by the Gordon and Betty Moore Foundation (grant to J.C.P.), the National Science Foundation (fellowship support for S.E.C.), and the Natural Sciences and Engineering Research Council of Canada (fellowship support for R.J.L.). Assistance with X-ray crystallography from Larry M. Henling is gratefully acknowledged.", revision_no = "14", abstract = "N-alkylations of carbazoles with a variety of secondary and hindered primary alkyl iodides can be achieved by using a simple precatalyst (CuI) under mild conditions (0\u2009°C) in the presence of a Brønsted base; at higher temperature (30\u2009°C), secondary alkyl bromides also serve as suitable coupling partners. A Li[Cu(carbazolide)_2] complex has been crystallographically characterized, and it may serve as an intermediate in the catalytic cycle.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/38659, title ="H–H and Si–H Bond Addition to Fe≡NNR_2 Intermediates Derived from N_2", author = "Suess, Daniel L. M. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "135", number = "13", pages = "4938-4941", month = "April", year = "2013", doi = "10.1021/ja400836u", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130523-114051847", note = "© 2013 American Chemical Society.\n\nReceived: January 24, 2013; Published: March 11, 2013. Published In Issue April 03, 2013. \n\nWe acknowledge the NIH (GM070757) and the Beckman Institute for funding and thank Lawrence Henling and Dr. Jens Kaiser for assistance with XRD studies. We also acknowledge the Gordon and Betty Moore Foundation, the Beckman Institute, and the Sanofi-Aventis BRP at Caltech for their support of the Molecular Observatory at Caltech. The SSRL is operated for the DOE and supported by its Office of Biological and Environmental Research, the NIH NIGMS (including P41GM103393), and the NCRR (P41RR001209).", revision_no = "24", abstract = "The synthesis and characterization of Fe–diphosphineborane complexes are described in the context of N_2 functionalization chemistry. Iron aminoimides can be generated at room temperature under 1 atm N_2 and are shown to react with E–H bonds from PhSiH_3 and H_2. The resulting products derive from delivery of the E fragment to Nα and the H atom to B. The flexibility and lability of the Fe–BPh interactions in these complexes engender this reactivity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/37931, title ="A Polar Copper−Boron One-Electron σ‑Bond", author = "Moret, Marc-Etienne and Zhang, Limei", journal = "Journal of the American Chemical Society", volume = "135", number = "10", pages = "3792-3795", month = "March", year = "2013", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130415-092609404", note = "© 2013 American Chemical Society. \n\nPublished In Issue March 13, 2013;\nArticle ASAP March 04, 2013;\nJust Accepted Manuscript February 18, 2013.\n\n\n\nThis work was supported by the Gordon and Betty Moore foundation. M.-E.M. acknowledges a Fellowship for Advanced Researchers from the Swiss National Science Foundation. L.Z. acknowledges a postdoctoral fellowship from Natural Sciences and Engineering Research Council of Canada. We acknowledge the Gordon and Betty Moore Foundation, the Beckman Institute, and the Sanofi-Aventis BRP at Caltech for their generous support of the Molecular Observatory at Caltech. SSRL is operated for the DOE and supported by its Office of Biological and Environmental Research, and by the NIH, NIGMS (including P41GM103393), and the NCRR (P41RR001209). The authors declare no competing financial interest.", revision_no = "26", abstract = "Virtually all chemical bonds consist of one or several pairs of electrons shared by two atoms. Examples of σ-bonds made of a single electron delocalized over two neighboring atoms were until recently found only in gas-phase cations such as H_2^+ and Li_2^+ and in highly unstable species generated in solid matrices. Only in the past decade was bona fide one-electron bonding observed for molecules in fluid solution. Here we report the isolation and structural characterization of a thermally stable compound featuring a Cu–B one-electron bond, as well as its oxidized (nonbonded) and reduced (two-electrons-bonded) congeners. This triad provides an excellent opportunity to study the degree of σ-bonding in a metalloboratrane as a function of electron count.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/37155, title ="Conversion of Fe−NH_2 to Fe−N_2 with release of NH_3", author = "Anderson, John S. and Moret, Marc-Etienne", journal = "Journal of the American Chemical Society", volume = "135", number = "2", pages = "534-537", month = "January", year = "2013", doi = "10.1021/ja307714m", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130226-145140419", note = "© 2012 American Chemical Society. \n\nReceived: August 3, 2012. Published: December 21, 2012. \n\nThe authors declare no competing financial interest. This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation, and through the NSF via a GRFP award to J.S.A. M.-E. M. acknowledges a Fellowship for Advanced Researchers from the Swiss National Science Foundation. Larry Henling and Dr. Jens Kaiser are thanked for their assistance with X-ray crystallography and Dr. Angelo DiBilio for his assistance with EPR measurements. We acknowledge the Gordon and Betty Moore Foundation, the Beckman Institute, and the Sanofi-Aventis BRP at Caltech for their generous support of the Molecular Observatory at Caltech. SSRL is operated for the DOE and supported by its Office of Biological and Environmental Research, and by the NIH, NIGMS (including P41GM103393), and the NCRR (P41RR001209). ", revision_no = "20", abstract = "Tris(phosphine)borane ligated Fe(I) centers featuring N_2H_4, NH_3, NH_2, and OH ligands are described. Conversion of Fe–NH_2 to Fe–NH_3^+ by the addition of acid, and subsequent reductive release of NH_3 to generate Fe–N_2, is demonstrated. This sequence models the final steps of proposed Fe–mediated nitrogen fixation pathways. The five-coordinate trigonal bipyramidal complexes described are unusual in that they adopt S = 3/2 ground states and are prepared from a four-coordinate, S = 3/2 trigonal pyramidal precursor.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35683, title ="Photoinduced Ullmann C–N Coupling: Demonstrating the Viability of a Radical Pathway", author = "Creutz, Sidney E. and Lotito, Kenneth J.", journal = "Science", volume = "338", number = "6107", pages = "647-651", month = "November", year = "2012", doi = "10.1126/science.1226458 ", issn = "0036-8075", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121127-143024844", note = "© 2012 American Association for the Advancement of Science. \n\nReceived 22 June 2012; accepted 10 September 2012. \n\nThis work was supported by the National Science Foundation (graduate research fellowships for S.E.C. and K.J.L.) and by the Gordon and Betty Moore Foundation. Metrical parameters for the structure of copper complex 1 are available free of charge from the Cambridge Crystallographic Data Centre under reference CCDC-896019.", revision_no = "15", abstract = "Carbon–nitrogen (C–N) bond-forming reactions of amines with aryl halides to generate arylamines (anilines), mediated by a stoichiometric copper reagent at elevated temperature (>180°C), were first described by Ullmann in 1903. In the intervening century, this and related C–N bond-forming processes have emerged as powerful tools for organic synthesis. Here, we report that Ullmann C–N coupling can be photoinduced by using a stoichiometric or a catalytic amount of copper, which enables the reaction to proceed under unusually mild conditions (room temperature or even –40°C). An array of data are consistent with a single-electron transfer mechanism, representing the most substantial experimental support to date for the viability of this pathway for Ullmann C–N couplings. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/35441, title ="Mononuclear Five- and Six-Coordinate Iron Hydrazido and Hydrazine Species", author = "Saouma, Caroline T. and Lu, Connie C.", journal = "Inorganic Chemistry", volume = "51", number = "18", pages = "10043-10054", month = "September", year = "2012", doi = "10.1021/ic301704f ", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121113-140054078", note = "© 2012 American Chemical Society. Received: August 2, 2012. Publication Date (Web): September 5, 2012. Dr. David VanderVelde, and Dr. Jeff Simpson are acknowledged\nfor insightful discussions regarding the NMR spectroscopy\nof the compounds. Dr. Peter Müller, Dr. Michael Day, and\nDr. Larry Henling are acknowledged for insightful discussions regarding X-ray crystallography. This work was generously supported by the NIH (GM-070757), and the Gordon and Betty Moore Foundation. Funding for the Caltech NMR facility was provided in part by the NIH (RR 027690). C.T.S. is grateful for an NSF graduate fellowship.", revision_no = "24", abstract = "This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N_2H_4), hydrazido (N_2H_3^–), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP^(mter)_3]^−, is introduced to provide access to species that cannot be stabilized with the [PhBP^(Ph)_3]^− ligand ([PhBP^R_3]^− = PhB(CH_2PR_2)_3^–). Treatment of [PhBP^(mter)_3]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP^(mter)_3]Fe(η^2-N_2H_3) (1), in which the hydrazido serves as an L_2X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP^(mter)_3]Fe(L)(η^2-N_2H_3) (L = N_2H_4 (2) or NH_3 (3)). In contrast, treatment of [PhBP^(Ph)_3]FeMe with hydrazine forms the adduct [PhBP^(Ph)_3]Fe(Me)(η^2-N_2H_4) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP^(Ph)_3]Fe(η^2-N_2H_3), which undergoes bimolecular coupling to produce {[PhBP^(Ph)_3]Fe}_2(μ-η^1:η^1-N_2H_4)(μ-η^2:η^2-N_2H_2). The oxidation of these and related hydrazine and hydrazido species is also presented. For example, oxidation of 1 or 5 with Pb(OAc)_4 results in disproportionation of the N_2H_x ligand (x = 3, 4), and formation of [PhBP^R_3]Fe(NH_3)(OAc) (R = Ph (9) and mter (11)).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/36156, title ="Access to formally Ni(I) states in a heterobimetallic NiZn system", author = "Uyeda, Christopher and Peters, Jonas C.", journal = "Chemical Science", volume = "4", number = "1", pages = "157-163", month = "September", year = "2012", doi = "10.1039/c2sc21231e ", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20130103-150617672", note = "© 2012 The Royal Society of Chemistry.\nReceived 10th August 2012.\nAccepted 10th September 2012.\nFirst published on the web 12 Sep 2012.\n\nThis work was supported by the NSF Center for Chemical\nInnovation: Powering the Planet grant CHE-0802907, and by the\nGordon and Betty Moore Foundation. We thank Larry Henling\nand Charlene Tsay for assistance with crystallography, Dr\nAngelo Di Bilio for EPR measurements, and Dr Limei Zhang and\nDr Jens Kaiser for XAS measurements. We acknowledge the\nGordon and Betty Moore Foundation, the Beckman Institute,\nand the Sanofi-Aventis BRP at Caltech for their generous\nsupport of the Molecular Observatory at Caltech. SSRL is operated\nfor the DOE and supported by its Office of Biological and\nEnvironmental Research, and by the NIH, NIGMS (including\nP41GM103393) and the NCRR (P41RR001209).", revision_no = "21", abstract = "Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have been synthesized using diimine–dioxime ligands as binucleating scaffolds. A tetramethylfuran-containing ligand derivative enables a stable one-electron-reduced S = 1/2 species to be accessed using Cp_2Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallographic characteristics of a ligand-centered radical bound to a Ni(II) center. Upon coordination of a π-acidic ligand such as PPh_3, however, a five-coordinate Ni(I) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the Ni^(II/I) couple for complexes of redox-active tetraazamacrocycles.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/33959, title ="Dihydrogen Binding to Isostructural S = 1/2 and S = 0 Cobalt Complexes", author = "Suess, Daniel L. M. and Tsay, Charlene", journal = "Journal of the American Chemical Society", volume = "134", number = "34", pages = "14158-14164", month = "August", year = "2012", doi = "10.1021/ja305248f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120910-104427509", note = "© 2012 American Chemical Society. \n\nReceived: May 30, 2012. Publication Date (Web): August 14, 2012. \n\nWe thank the Gordon and Betty Moore Foundation and the NSF Center for Chemical Innovation on Solar Fuels (CCI Solar, CHE-0802907 and CHE-0947829) for funding. We thank Prof. George Rossman (solid-state Raman studies), Maraia Ener (solution-state Raman studies), and Lawrence Henling (X-ray crystallography) for experimental assistance.", revision_no = "21", abstract = "Two isostructural, nonclassical Co(H_2) complexes are prepared from their Co(N_2) precursors using tris(phosphino)silyl and tris(phosphino)borane ancillary ligands. Comproportionation of CoBr_2 and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)borane) gives (TPB)CoBr (4). One-electron reduction of 4 triggers N_2 binding to give (TPB)Co(N_2) (2-N_2) which is isostructural to previously reported [SiP_3]Co(N_2) (1-N_2) ([SiP_3] = tris-(o-diisopropylphosphinophenyl)silyl). Both 1-N_2 and 2-N_2 react with 1 atm H_2 to generate thermally stable H_2 complexes 1-H_2 and 2-H_2, respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H_2 and N_2 ligands in 2-H_2 and 2-N_2 are labile under ambient conditions and the binding equilibria are observable by temperature-dependent UV/vis. A van’t Hoff analysis allows for the ligand binding energetics to be determined (H_2: ΔHº = −12.5(3) kcal mol^(–1) and ΔSº = −26(3) cal K^(–1) mol^(–1); N_2: ΔHº = −13.9(7) kcal mol^(–1) and ΔSº = −32(5) cal K^(–1) mol^(–1)).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/34801, title ="Late-Metal Diphosphinosulfinyl S(O)P_2 Pincer-type Complexes", author = "Suess, Daniel L. M. and Peters, Jonas C.", journal = "Organometallics", volume = "31", number = "15", pages = "5213-5222", month = "August", year = "2012", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20121009-153825920", note = "© 2012 American Chemical Society. Received: February 14, 2012;\nPublished: May 31, 2012.\nThe authors declare no competing financial interest. This work was funded by generous support from the NSF (CHE-0750234) and the Gordon and Betty Moore Foundation. We thank Larry Henling for assistance with crystallography. D.L.M.S. acknowledges an NSF Graduate Research Fellowship. ", revision_no = "17", abstract = "The preparation of a new tridentate diphosphinosulfinyl ligand is described as well as the synthesis and properties of some of its Rh, Ir, Ni, Pd, and Pt complexes. The ligand binds in a κ^3-PS(O)P fashion in all cases. The M–S lengths in (SOP_2)RhCl and (SOP_2)IrCl (2.1340(8) and 2.1341(5) Å, respectively) are the shortest of all crystallographically characterized Rh– and Ir–S(O)R_2 complexes, which illustrates the significant M–S(O) π-backbonding for these metals. Akin to Vaska’s complex, (SOP_2)IrCl binds O_2 to yield a peroxide complex with an O–O length of 1.465(3) Å and ^ν(O–O) = 847 cm^(–1). The C–O stretches of (SOP_2)M(CO) (M = Ni, Pd, Pt) are ~30-40 cm^(–1) higher than those of the analogous (PPh_3)_3M(CO). A number of divalent group 10 complexes of the form [(SOP_2)MX]^+ for Ni (X = Cl), Pd (X = Cl, Me), and Pt (X = Cl, Me) are reported, as well as the highly electrophilic complex [(SOP_2)Pd(NCCH_3)]^(2+). An analysis of the S–O length as a function of the M–S length for all d^8 SOP_2 Rh, Ir, Pd, and Pt complexes reveals that the electron-withdrawing capabilities of the sulfinyl group are mediated largely through π-backbonding for Rh and Ir and mostly through poor σ-donation for Pd and Pt.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/34070, title ="Modeling the Signatures of Hydrides in Metalloenzymes: ENDOR Analysis of a Di-iron Fe(μ-NH)(μ-H)Fe Core", author = "Kinney, R. Adam and Saouma, Caroline T.", journal = "Journal of the American Chemical Society", volume = "134", number = "30", pages = "12637-12647", month = "August", year = "2012", doi = "10.1021/ja303739g ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120913-131405418", note = "© 2012 American Chemical Society. \n\nReceived: April 18, 2012. Published: July 23, 2012. \n\nThis work was supported by the NIH (HL13531, B.M.H.; GM070757, J.C.P.). C.T.S. was supported by an NSF Graduate Fellowship. We thank Dr. Peter Doan for helpful conversations, and Mr. Clark Davoust for his invaluable technical expertise.", revision_no = "26", abstract = "The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L_3Fe(μ-NH)(μ-H)FeL_3 (L_3 = [PhB(CH_2PPh_2)_3]−) complex, 3, is a novel S = 1/2 type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. ^(1,2)H and ^(14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a ‘bridging’ hydride that is essentially equidistant between two ‘anchor’ metal ions: a rhombic dipolar interaction tensor, T ≈ [T, –T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the ‘anchor’ metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [−T, –T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum–iron cofactor of nitrogenase contains two [Fe–H––Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E_4). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E_4. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/31456, title ="A Ru(I) Metalloradical That Catalyzes Nitrene Coupling to Azoarenes from Arylazides", author = "Takaoka, Ayumi and Moret, Marc-Etienne", journal = "Journal of the American Chemical Society", volume = "134", number = "15", pages = "6695-6706", month = "April", year = "2012", doi = "10.1021/ja211603f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120514-132407274", note = "© 2012 American Chemical Society. \n\nReceived: December 12, 2011; Published: March 1, 2012. \n\nWe acknowledge the generosity of NSF (Grant CHE-0750234) and the Gordon and Betty Moore Foundation for support of this work. M.-E.M. acknowledges a Fellowship for Advanced Researchers from the Swiss National Science Foundation. We also thank Dr. Hill Harman for insightful discussions. Dr. Peter Müller, Larry Henling, and Charlene Tsay provided crystallographic assistance.", revision_no = "27", abstract = "Unusual N–N coupling of aryl azides to yield azoarenes is demonstrated by the Ru(I) metalloradical, [SiP^(iPr)_3]Ru(N_2) (4) ([SiP^(iPr)_3] = (2-iPr_(2)PC_(6)H_(4))_(3)Si^–). The yield of the azoarene is dependent on the substituent on the aryl azide, and the reaction is catalytic for p-methoxy and p-ethoxy phenyl azides, while no azoarene is observed for p-trifluoromethylphenyl azide. Studies aimed at probing the viability of a bimolecular coupling mechanism of metal imide species, as shown in the related [SiP^(iPr)_3]Fe system, have led to the isolation of several structurally unusual complexes including the ruthenium(IV) imide, 7-OMe, as well as the Ru(II) azide adduct 8-OMe. One electron reduction of 7-OMe complex led to the isolation of the formally Ru(III) imide complex, [SiP^(iPr)_3]Ru(NAr) (Ar = p-MeOC_(6)H_(4), 5-OMe). EPR spectroscopy on 5-OMe suggests that the complex is electronically similar to the previously reported imide complex, [SiP^(iPr)_3]Ru(NAr) (Ar = p-CF_(3)C_(6)H_(4),5-CF_(3)), and features radical character on the NAr moiety, but to a greater degree. The stability of 5-OMe establishes that bimolecular coupling of 5-OMe is kinetically inconsistent with the reaction. Further studies rule out mechanisms in which 5-OMe reacts directly with free aryl azide or a transient Ru(I) azide adduct. Together, these studies show that 5-OMe is likely uninvolved in the catalytic cycle and demonstrates the influence of the metal center on the mechanism of reaction. Instead, we favor a mechanism in which free aryl nitrene is released during the catalytic cycle and combines with itself or with free aryl azide to yield the azoarene.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47918, title ="Delayed fluorescence of a dinuclear copper(I) complex", author = "Deaton, Joseph C. and Switalski, Steven C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "243", pages = "INOR 1057", month = "March", year = "2012", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-141302088", note = "© 2012 American Chemical Society.", revision_no = "11", abstract = "A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was\nshown to exhibit E-type delayed fluorescence by variable temp. emission spectroscopy and\nphotoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet\nexcited states were assigned as charge transfer states on the basis of theor. calcns. and the small obsd. S_1-T_1\nenergy gap. Vapor deposited OLEDs doped with the complex in the emissive layer gave a max. external\nquantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the\ndelayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several\nphys. methods, including elec. detected EPR, cyclic voltammetry, and photoluminescence in the presence of\napplied current. Work carried out at Eastman Kodak Company, Rochester, NY 14650, was done prior to Dec.,\n2009.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47916, title ="Ru(I) metalloradical that catalyzes nitrene coupling to azoarenes from arylazides", author = "Takaoka, Ayumi and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "243", pages = "INOR 1011", month = "March", year = "2012", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-140126924", note = "© 2012 American Chemical Society.", revision_no = "11", abstract = "Our group recently studied the interaction between aryl azides and the Fe(I) complex, [SiP^(iPr)_3]Fe(N_2) ([SiP^(iPr)_3] = ([SiP^(iPr)_3] = (2-iPr_2PC_6H_4)_3Si-). This reaction was found to initially form an Fe(I) azide adduct, [SiP^(iPr)_3]Fe(N_3Ar), which subsequently exhibited clean unimol. decay. Interestingly, the major product of this decay was shown to be azoarene and the starting complex [SiP^(iPr)_3]Fe(N_2), and use of excess aryl azide demonstrated catalytic azoarene formation from [SiP^(iPr)_3]Fe(N_2). While several related stoichiometric reactions had been known, this example was noteworthy in that it represented a rare example of catalytic N-N coupling to yield azoarene from org. azides. Mechanistic studies suggested the formation of a transient Fe(III) imide complex, [SiP^(iPr)_3]Fe(NAr), following decay of [SiP^(iPr)_3]Fe(N_3Ar), which subsequently underwent 4e- reductive N-N coupling to produce azoarene. [SiP^(iPr)_3]Fe(NAr) is a reactive S = 1/2 species that was only observable by EPR spectroscopy in a frozen glass. In this regard, the recent isolation of the Ru(I) metalloradical, [SiP^(iPr)_3] Ru(N_2), and its interaction with p-CF_3C_6H_4N_3to yield the formally Ru(III) imide complex, [SiP^(iPr)_3]Ru(NC_6H_4CF_3), is noteworthy. [SiP^(iPr)_3]Ru(NC_6H_4CF_3) did not yield azoarene upon decay, and was stable enough for thorough characterization. We thus envisioned that use of other substituted aryl azides might yield similar metal imide species that would retain some stability for characterization, yet also exhibit N-N coupling reactivity as obsd. in the Fe system. In this talk I will report on recent results of our studies on [SiP^(iPr)_3]Ru(N_2) with substituted aryl azides. The tendency of the aryl azide to degrade in the presence of [SiP^(iPr)_3]Ru(N_2) to azoarene product, either stoichiometrically or catalytically, is dependent on the aryl-ring substitution pattern.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/31601, title ="Reversible H_2 Addition across a Nickel−Borane Unit as a Promising Strategy for Catalysis", author = "Harman, W. Hill and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "134", number = "11", pages = "5080-5082", month = "March", year = "2012", doi = "10.1021/ja211419t ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120523-074508665", note = "© 2012 American Chemical Society. Received: December 6, 2011. Publication Date (Web): March 1, 2012. This work was supported by the NSF Center for Chemical Innovation: Powering the Planet grant CHE-0802907, and by the Gordon and Betty Moore Foundation. We thank Professor Greg Fu for a helpful suggestion.", revision_no = "27", abstract = "We report the synthesis and characterization of a series of nickel complexes of the chelating diphosphine-borane ligands ArB(o-Ph_2PC_6H_4)_2 ([^(Ar)DPB^(Ph)]; Ar = Ph, Mes). The [^(Ar)DPB^(Ph)] framework supports pseudo-tetrahedral nickel complexes featuring η^2-B,C coordination from the ligand backbone. For the B-phenyl derivative, the THF adduct [^(Ph)DPB^(Ph)]Ni(THF) has been characterized by X-ray diffraction and features a very short interaction between nickel and the η^2-B,C ligand. For the B-mesityl derivative, the reduced nickel complex [^(Mes)DPB^(Ph)]Ni is isolated as a pseudo-three-coordinate “naked” species that undergoes reversible, nearly thermoneutral oxidative addition of dihydrogen to give a borohydrido-hydride complex of nickel(II) which has been characterized in solution by multinuclear NMR. Furthermore, [^(Mes)DPB^(Ph)]Ni is an efficient catalyst for the hydrogenation of olefin substrates under mild conditions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/30135, title ="Electrocatalytic Hydrogen Evolution in Acidic Water with Molecular Cobalt Tetraazamacrocycles", author = "McCrory, Charles C. L. and Uyeda, Christopher", journal = "Journal of the American Chemical Society", volume = "134", number = "4", pages = "3164-3170", month = "February", year = "2012", doi = "10.1021/ja210661k", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120417-113331818", note = "© 2012 American Chemical Society.\n\nPublished In Issue February 15, 2012; Article ASAP February 06, 2012; Just Accepted Manuscript January 12, 2012; Received: November 12, 2011.\n\nFinancial support for this work was provided by an NSF Center for Chemical Innovation (CHE-0802907).", revision_no = "24", abstract = "A series of water-soluble molecular cobalt complexes of tetraazamacrocyclic ligands are reported for the electrocatalytic production of H2 from pH 2.2 aqueous solutions. The comparative data reported for this family of complexes shed light on their relative efficiencies for hydrogen evolution in water. Rotating disk electrode voltammetry data are presented for each of the complexes discussed, as are data concerning their respective pH-dependent electrocatalytic activity. In particular, two diimine–dioxime complexes were identified as exhibiting catalytic onset at comparatively low overpotentials relative to other reported homogeneous cobalt and nickel electrocatalysts in aqueous solution. These complexes are stable at pH 2.2 and produce hydrogen with high Faradaic efficiency in bulk electrolysis experiments over time intervals ranging from 2 to 24 h.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/29973, title ="Thermally stable N_2 and H_2 adducts of cationic nickel(II)", author = "Tsay, Charlene and Peters, Jonas C.", journal = "Chemical Science", volume = "3", number = "4", pages = "1313-1318", month = "January", year = "2012", issn = "2041-6520", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120404-092548200", note = "© 2012 The Royal Society of Chemistry. Received 8th December 2011, Accepted 18th January 2012. First published on the web 19 Jan 2012. This work was supported by the NSF (CHE-0750234) and the Gordon and Betty Moore Foundation. We are grateful to Dr Peter Müller and Dr Marc-Etienne Moret for assistance with\nXRD and DFT studies, respectively.", revision_no = "25", abstract = "The first examples of thermally stable molecular dihydrogen adducts of nickel were synthesized from their dinitrogen adduct precursors, which are themselves among the first examples of Ni(II)-N_2 complexes. The minimal activation of the bound N_2 moieties suggests that these adducts are stabilized predominantly through σ-donation from the adduct to the electrophilic metal center. We further show that the bound H_2 ligand can undergo heterolytic cleavage to deliver hydride to the nickel center. The H_2 adducts are of particular interest in the context of hypotheses suggesting that Ni can serve as the site for H_2 binding and heterolytic activation in [NiFe] hydrogenases.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/29275, title ="A Homologous Series of Cobalt, Rhodium, and Iridium\nMetalloradicals", author = "Takaoka, Ayumi and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "51", number = "1", pages = "16-18", month = "January", year = "2012", doi = "10.1021/ic202079r", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120214-104203697", note = "© 2011 American Chemical Society. Received: September 23, 2011. Publication Date (Web): December 1, 2011. This work was supported by the NSF (Grant CHE-0750234). Charlene Tsay and Larry Henling are acknowledged for crystallographic assistance.", revision_no = "28", abstract = "We herein present a series of d7 trimethylphosphine complexes of group 9 metals that are chelated by the tripodal tetradentate tris(phosphino)silyl ligand [SiP^(iPr)_3]H ([SiP^(iPr)_3] = (2_(-i)Pr_2PC_6H_4)_3Si^–). Both electron paramagnetic resonance (EPR) simulations and density functional theory (DFT) calculations indicate largely metalloradical character. These complexes provide a rare opportunity to compare the properties between the low-valent metalloradicals of the second- and third-row transition metals with the corresponding first-row analogues.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28525, title ="A Five-Coordinate Phosphino/Acetate Iron(II) Scaffold That Binds N₂, N₂H₂, N₂H₄, and NH₃ in the Sixth Site", author = "Saouma, Caroline T. and Moore, Curtis E.", journal = "Inorganic Chemistry", volume = "50", number = "22", pages = "11285-11287", month = "November", year = "2011", doi = "10.1021/ic2016066 ", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111219-151527805", note = "© 2011 American Chemical Society. \n\nReceived: July 26, 2011. Published: October 17, 2011. Publication Date (Web): October 17, 2011. \n\nWe acknowledge the NIH (Grant GM-070757). Funding for the Caltech NMR facility has been provided, in part, by the NIH (Grant RR027690). C.T.S. is grateful for an NSF graduate fellowship.", revision_no = "33", abstract = "A family of iron(II) complexes that coordinate dinitrogen, diazene, hydrazine, and ammonia are presented. This series of complexes is unusual in that the complexes within it feature a common auxiliary ligand set and differ only by virtue of the nitrogenous N_xH_y ligand that occupies the sixth binding site. The ability of an iron center to bind N₂, N₂H₂, N₂H₄, and NH₃ is important to establish in the context of evaluating catalytic N₂ reduction schemes that invoke these nitrogenous species. Such a scenario has been proposed as an iron-mediated, alternating reduction scheme within the cofactor of nitrogenase enzymes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28664, title ="N_2 Functionalization at Iron Metallaboratranes", author = "Moret, Marc-Etienne and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "133", number = "45", pages = "18118-18121", month = "November", year = "2011", doi = "10.1021/ja208675p", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120105-085838649", note = "© 2011 American Chemical Society. Received: September 14, 2011. Publication Date (Web): October 18, 2011. This work was supported by the NIH (GM 070757). M.-E.M.\nacknowledges a Fellowship for Prospective Researchers from the Swiss National Science Foundation. We thank Charlene Tsay and Larry Henling for crystallographic assistance, as well Angelo Di Bilio for assistance with EPR measurements.", revision_no = "27", abstract = "The reactivity of the anionic dinitrogen complex [(TPB)Fe(N_2)]^− (TPB = tris[2-(diisopropylphosphino)phenyl]borane) toward silicon electrophiles has been examined. [(TPB)Fe(N_2)]^− reacts with trimethylsilyl chloride to yield the silyldiazenido complex (TPB)Fe(NNSiMe_3), which is reduced by Na/Hg in THF to yield the corresponding sodium-bound anion [(TPB)Fe(NNSiMe_3)]Na(THF). The use of 1,2-bis(chlorodimethylsilyl)ethane in the presence of excess Na/Hg results in the disilylation of the bound N_2 molecule to yield the disilylhydrazido(2−) complex (TPB)Fe≡NR (R = 2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentyl). One of the phosphine arms of TPB in (TPB)Fe≡NR can be substituted by CO or ^tBuNC to yield crystalline adducts (TPB)(L)Fe≡NR (L = CO, ^tBuNC). The N–N bond in (TPB)(^tBuNC)Fe≡NR is cleaved upon standing at room temperature to yield a phosphoraniminato/disilylamido iron(II) complex. The flexibility of the Fe–B linkage is thought to play a key role in these transformations of Fe-bound dinitrogen.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28696, title ="Rapid Water Reduction to H_2 Catalyzed by a Cobalt Bis(iminopyridine) Complex", author = "Stubbert, Bryand D. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "133", number = "45", pages = "18070-18073", month = "November", year = "2011", doi = "10.1021/ja2078015", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120106-104425949", note = "© 2011 American Chemical Society. Received: August 17, 2011. Publication Date (Web): October 24, 2011. We thank Charles C. L. McCrory, Christopher Uyeda, and\nJames R. McKone for helpful discussions. Our work was supported by the NSF Center for Chemical Innovation on Solar\nFuels (CCI Solar, CHE-0802907 and CHE-0947829) and the\nStanford Global Climate and Energy Project (GCEP).", revision_no = "25", abstract = "A cobalt bis(iminopyridine) complex is a highly active electrocatalyst for water reduction, with an estimated apparent second order rate constant k_(app) ≤ 10^7 M^(–1)s^(–1) over a range of buffer/salt concentrations. Scan rate dependence data are consistent with freely diffusing electroactive species over pH 4–9 at room temperature for each of two catalytic reduction events, one of which is believed to be ligand based. Faradaic H_2 yields up to 87 ± 10% measured in constant potential electrolyses (−1.4 V vs SCE) confirm high reactivity and high fidelity in a catalyst supported by the noninnocent bis(iminopyridine) ligand. A mechanism involving initial reduction of Co^(2+) and subsequent protonation is proposed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27794, title ="A Nonclassical Dihydrogen Adduct of S = ½ Fe(I)", author = "Lee, Yunho and Kinney, R. Adam", journal = "Journal of the American Chemical Society", volume = "133", number = "41", pages = "16366-16369", month = "October", year = "2011", doi = "10.1021/ja207003m", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111116-075831871", note = "© 2011 American Chemical Society. \n\nReceived: July 26, 2011. Publication Date (Web): September 28, 2011. \n\nWe acknowledge the NIH (GM-070757 to J.C.P.; HL 13531 to B.M.H.). We thank Prof. Harden McConnell for insightful suggestions about low-temperature dynamics. Dr. Peter Müller provided assistance with XRD analyses. Professor George R. Rossman provided access to a near-IR spectrometer.Henry Fong\nprobed the deprotonation of 5.", revision_no = "35", abstract = "We have exploited the capacity of the “(SiP^(iPr)_3)Fe(I)” scaffold to accommodate additional axial ligands and characterized the mononuclear S = 1/2 H_2 adduct complex (SiP^(iPr)_3)Fe^I(H_2). EPR and ENDOR data, in the context of X-ray structural results, revealed that this complex provides a highly unusual example of an open-shell metal complex that binds dihydrogen as a ligand. The H2 ligand at 2 K dynamically reorients within the ligand-binding pocket, tunneling among the energy minima created by strong interactions with the three Fe–P bonds.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47915, title ="Homogeneous cobalt electrocatalysts for solar driven hydrogen evolution from water", author = "Stubbert, Bryan D. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "242", pages = "INOR 489", month = "August", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-135735898", note = "© 2011 American Chemical Society.", revision_no = "12", abstract = "Towards the bigger picture of developing modular solns. for photoelectrochem. water splitting, catalytic proton\nredn. mediated by cobalt bis(iminopyridine) complexes has been explored in buffered aq. media over a broad\nrange of pH conditions. The extremely rapid rates of electrocatalytic hydrogen evolution exhibited by firstgeneration\ncatalysts will be discussed in conjunction with the performance of second-generation\nelectrocatalysts. Details of electrochem. and photochem. mechanistic studies, kinetic competency, and the\neffects of reaction medium on H_2 evolution will also be presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/25416, title ="Phosphido pincer complexes of platinum: synthesis, structure and reactivity", author = "Mazzeo, Mina and Strianese, Maria", journal = "Dalton Transactions", volume = "40", number = "35", pages = "9026-9033", month = "August", year = "2011", doi = "10.1039/c1dt10825e", issn = "1477-9226", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110923-083810544", note = "© 2011 The Royal Society of Chemistry. Received 03 May 2011, Accepted 27 Jul 2011. First published on the web 09 Aug 2011. This article is part of the collection: Pincers and other hemilabile ligands. The authors thank Dr Neal P. Mankad and Dr Matthew T. Whited for assistance with crystallographic studies (Institute of Technology, Pasadena, California), Patrizia Oliva for NMR assistance (University of Salerno) and Dr Consiglia Tedesco for\nuseful discussions. JCP acknowledges CHE-0750234 for financial support. MM acknowledges FARB 2008 for financial support. ", revision_no = "26", abstract = "A series of platinum(II) complexes supported by the tridentate bis(phosphine)phosphido ligand bis(2-diisopropylphosphinophenyl)phosphide) [iPr–PPP] have been synthesized and characterized (1–4). X-Ray structural studies of [iPr–PPP]PtCl (1) and [iPr–PPP]PtCH3 (3) complexes show meridional [iPr–PPP] ligands around approximately square-planar platinum centers. Structural data and NMR analysis highlight a strong trans influence for the phosphido phosphorous donor, comparable to that of the anionic aryl carbon of the classic PCP pincer complexes. A series of thermally stable [PPP]Pt(IV) compounds, including [PPP]Pt(CH_3)_2X [X = I (5) and SbF_6 (6)], were also synthesized. The study of the binding affinity of SO_2 and NO to complex 1 has also been addressed.\n\n\n\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/24235, title ="Dinitrogen Complexes of Sulfur-Ligated Iron\n", author = "Takaoka, Ayumi and Mankad, Neal P.", journal = "Journal of the American Chemical Society", volume = "133", number = "22", pages = "8440-8443", month = "June", year = "2011", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110628-093114193", note = "© 2011 American Chemical Society. Received: March 7, 2011.\nPublished: May 16, 2011. This work was supported by the NIH (GM070757). Charlene Tsay and Larry Henling are acknowledged for crystallographic assistance. Prof. George Rossman is acknowledged for use of his NIR spectrometer. Dr. Dave Harris is acknowledged for useful discussions of the magnetic data.", revision_no = "26", abstract = "We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R_2PC_6H_4)_3Si (R = Ph, iPr) but have one or two phosphine arms replaced with thioether donors. Depending on the number of phosphine arms replaced, both mononuclear and dinuclear iron complexes with dinitrogen are accessible. These complexes contribute to a desirable class of model complexes that possess both dinitrogen and sulfur ligands in the immediate iron coordination sphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23236, title ="Transformation of an [Fe(η^2-N_2H_3)]^+ Species to π-Delocalized [Fe_2(μ-N_2H_2)]^(2+/+) Complexes", author = "Saouma, Caroline T. and Kinney, R. Adam", journal = "Angewandte Chemie International Edition", volume = "50", number = "15", pages = "3446-3449", month = "April", year = "2011", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110404-112857625", note = "© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.\n\nReceived: October 7, 2010; Revised: November 30, 2010.\n\nWe acknowledge the NIH (GM-070757, J.C.P.; HL 13531, B.M.H.) and the NSF (MCB0723330, B.M.H.). Funding for the Caltech NMR facility has been provided in part by the NIH (RR 027690) and funding for the MIT Department of Chemistry Instrumentation Facility has been provided in part by the NSF (CHE-0234877). The Betty and Gordon Moore Foundation supports the Molecular Observatory at Caltech. C.T.S. is grateful for an NSF graduate fellowship. Alec Durrell and Jens Kaiser provided guidance for resonance Raman and XRD experiments, respectively.", revision_no = "17", abstract = "A monomeric Fe(η^2-N_2H_3) species has been prepared, and exposure to oxygen yields a diiron complex that features five-coordinate iron centers and an activated bridging diazene ligand (see picture; C gray, N blue, Fe black, P red, O green). Structural, theoretical, and spectroscopic data for the redox pair [Fe_2(μ-N_2H_2)]^(2+/+) are consistent with 4-center, 4-electron π-delocalized bonding across the Fe-NH-NH-Fe core that finds analogy in butadiene and the butadiene anion.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23574, title ="M≡E and M=E complexes of iron and cobalt that emphasize three-fold symmetry (E≡O, N, NR)", author = "Saouma, Caroline T. and Peters, Jonas C.", journal = "Coordination Chemistry Reviews", volume = "255", number = "7-8", pages = "920-937", month = "April", year = "2011", doi = "10.1016/j.ccr.2011.01.009", issn = "0010-8545", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110506-110257061", note = "© 2011 Elsevier B.V. \n\nReceived 5 November 2010; accepted 4 January 2011. Available online 13 January 2011. \n\nThe authors are indebted to our many co-workers and colleagues in the field that have contributed to the work that has been described in this review. JCP is particularly grateful to those students and postdocs that shaped the intellectual principles that now form the basis of our ideas concerning L3M(E) and LL3M(E) systems of iron and cobalt. Wealso wish to acknowledge the various sources of funding that have, over the years, made the relevant work in our labs possible. This includes support from the NIH (GM-070757), the NSF (CHE-0132216 and 0750234), the Dreyfus Foundation, and the Alfred P. Sloan Foundation. CTS is grateful for an NSF Graduate Fellowship.", revision_no = "17", abstract = "Mid-to-late transition metal complexes that feature terminal, multiply bonded ligands such as oxos, imides, and nitrides have been invoked as intermediates in several catalytic transformations of synthetic and biological significance. Until about ten years ago, isolable examples of such species were virtually unknown. Over the past decade or so, numerous chemically well-defined examples of such species have been discovered. In this context, the present review summarizes the development of 4- and 5-coordinate Fe(E) and Co(E) species under local three-fold symmetry.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/24296, title ="Silylation of Iron-Bound Carbon Monoxide Affords a Terminal Fe Carbyne", author = "Lee, Yunho and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "133", number = "12", pages = "4438-4446", month = "March", year = "2011", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110705-113849042", note = "© 2011 American Chemical Society.\n\nPublished In Issue March 30, 2011; Article ASAP March 04, 2011; Received: October 27, 2010.\n\nWe acknowledge the NSF (CHE-0750234) for financial support. Dr. Neal P. Mankad provided assistance with XRD analyses, and Prof. Theodore A. Betley provided access to a\nMössbauer spectrometer at Harvard University. Prof. Michael T. Green and Dr. Marc-Etienne Moret are acknowledged for\ninsightful discussions concerning theoretical data. We also want to express gratitude to a reviewer for suggestions that improved the reported X-ray data.", revision_no = "26", abstract = "A series of monocarbonyl iron complexes in the formal oxidation states 0, +1, and +2 are accessible when supported by a tetradentate tris(phosphino)silyl ligand (SiP^(iPr)_3 = [Si(o-C_(6)H_(4)PiPr_(2))_3]−). X-ray diffraction (XRD) studies of these carbonyl complexes establish little geometrical change about the iron center as a function of oxidation state. It is possible to functionalize the terminal CO ligand of the most reduced carbonyl adduct by addition of SiMe_(3)^+ to afford a well-defined iron carbyne species, (SiP^(iPr)_3)Fe≡C—OSiMe_3. Single-crystal XRD data of this iron carbyne derivative reveal an unusually short Fe≡C—OSiMe_3 bond distance (1.671(2) Å) and a substantially elongated C−O distance (1.278(3) Å), consistent with Fe−C carbyne character. The overall trigonal bipyramidal geometry of (SiP^(iPr)_3)Fe≡C—OSiMe_3 compares well with that of the corresponding carbonyls, (SiP^(iPr)_3)Fe(CO)^−, (SiP^(iPr)_3)Fe(CO), and (SiP^(iPr)_3)Fe(CO)^+. Details regarding the electronic structure of the carbyne complex have been explored via the collection of comparative Mössbauer data for all of the complexes featured and also via DFT calculations. In sum, these data point to a strongly π-accepting Fischer-type carbyne ligand that confers stability to a low-valent iron(0) rather than high-valent iron(IV) center.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47794, title ="Borane-linked diphosphine complexes towards dual activation of substrates relevant to energy catalysis", author = "Harman, W. H. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 938", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-105722456", note = "© 2011 American Chemical Society.", revision_no = "11", abstract = "In order to minimize the activation barriers assocd. with the activation of inert small mols. such as carbon\ndioxide and water, biol. enzymes frequently employ multiple simultaneous modes of substrate interaction,\nincluding coordination to multiple metal sites and/or strategically placed hydrogen bonding functionalities. In\nparticular, carbon dioxide is a prime candidate for activation by both Lewis acids and bases, as both natural\nenzymes and recent work in frustrated Lewis pair chem. have demonstrated. Herein we disclose a series of\ncompds. designed to deploy an analogous strategy towards carbon monoxide fixation in which a redox-active\ntransition metal serves as the Lewis base, and a pendant borane as the Lewis acid.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47765, title ="Entry into the dinitrogen chemistry of sulfur-ligated iron complexes", author = "Takaoka, Ayumi and Mankad, Neal", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 122", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-083853713", note = "© 2011 American Chemical Society.", revision_no = "10", abstract = "Although many examples of dinitrogen complexes of transition metals have been reported to date, few\ncontain sulfur-based ligands, and those of iron are particularly uncommon. Such complexes are relevant\ntowards constructing functional and structural models of the FeMo cofactor in nitrogenase, which catalytically\nreduces dinitrogen to ammonia through a cycle in which dinitrogen binding at iron has been implicated.\nThis presentation describes the synthesis of dinitrogen complexes of iron chelated by the tripodal\ntetradentate ligands, [SiP^R_Xs^(R')_y-]- ([SiP^R_xS^(R')_y- = (R_2PC_6H_4)_x(R'SC_6H_4)_ySi- : R = iPr, Ph; R' = Ad), which contain\none or two thioether arms complemented by phosphine arms. The characterization of several Fe(II) and Fe(I)\ncomplexes is presented, and a comparison with the iron dinitrogen chem. of the parent tris(phosphino)silyl\nligands, [SiP^R_3]- (SiP^R_3 = (R_2PC_6H_4)_3Si-; R = iPr, Ph) is discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47786, title ="Iron complexes of a tris(phosphino)borane ligand as nitrogenase models", author = "Moret, Marc-Etienne and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 935", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-104434319", note = "© 2011 American Chemical Society.", revision_no = "10", abstract = "Biol. dinitrogen fixation - i.e. The redn. of N_2 to two equiv. of ammonia by addn. of protons and electrons - is\nthought to proceed through initial coordination of N_2 to one or more iron centers of the nitrogenase MoFe\ncofactor. To shed light on the mechanism of this transformation, a no. of model compds. have previously been\nprepd. that mimic the trigonal environment of the iron center in the MoFe cofactor by enforcing either\npseudotetrahedral (PT) or trigonal-bipyramidal (TBP) geometry with robust phosphine-based multidentate\nligands. In this contribution, we show that both geometries can be accessed in ferraboratranes derived from a\ntris(phosphino)borane ligands. In particular, we describe terminal iron-N_2 complexes that exhibit a TBP\ngeometry as well as PT compds. with a Fe≡NR triple bond. Efforts towards the functionalization of the ironbound dinitrogen mol. are also discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47784, title ="Synthesis and reactivity of late transition metal complexes bearing a pincer-type diphosphinesulfoxide ligand", author = "Suess, Daniel L. M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 1121", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-104149023", note = "© 2011 American Chemical Society.", revision_no = "10", abstract = "A new pincer-type diphosphinesulfoxide ligand (\"SOP_2\") has been synthesized and complexed to a no. of middle and late transition metals. The central sulfoxide donor behaves as a σ donor and a Π acceptor, which has been chem. Demonstrated by the prepn. of (SOP_2)IrCl and its ability to bind dioxygen analogously to Vaska's complex. The suitability of this ligand architecture to accommodate both low-valent group 9 and 10 metal centers as well as high-valent centers with strongly Π-donating ligands trans to the sulfoxide donor will be the primary focus of discussion.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47870, title ="Synthesis and reactivity of novel iron complexes supported by neutral tris(phosphino)borane ligands", author = "MacMillan, Samantha N. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 941", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-083941677", note = "© 2011 American Chemical Society.", revision_no = "12", abstract = "Of central importance to elucidating iron's role in biol. nitrogen fixation are the accessible geometries and\noxidn. states of proposed iron intermediates. Our group has an ongoing interest in exploring the chem. of Fe-\nN_xH_y species and have successfully described terminal imide and nitride species supported by anionic\ntris(phosphino)borate ligands as well as terminal N_2 complexes stabilized by anionic tris(phosphino)silyl\nligands in pseudo-tetrahedral and trigonal-bipyramidal geometries, resp. A single ligand scaffold capable of\nstabilizing low-valent complexes with a π-acidic N_2 mol., as well as high-valent complexes with a π-basic\nnitride ligand would certainly be of interest; accordingly, the synthesis of a series of iron complexes\nsupported by the neutral tris(phosphino)borane ligand, [TPB^R] ([TPB^R] = (o-(R_2P)C_6H_4)_3B), R = iPr, Ph) will\nbe presented and their structures and chem. reactivity discussed within the context of the hemi-lability of the\napical Lewis-acidic borane.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47871, title ="Synthesis and redox reactivity of a series of Fe2(µ-S) complexes", author = "Anderson, John S. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 932", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-084245064", note = "© 2011 American Chemical Society.", revision_no = "11", abstract = "Reaction of (PhBP_3)FeCl (PhBP_3 = [PhB(CH_2PPh_2)_3]-) with [NBu_4][SH] and a base yields the dimeric bridging\nsulfide species [(PhBP_3)Fe]_2(μ-S). Cyclic voltammetry reveals two reversible redn. events spanning three\ndistinct oxidn. states from Fe(II)/Fe(II) to Fe(I)/Fe(I). The mixed valent {[(PhBP_3)Fe]_2(μ-S)}- and the dianionic {[(PhBP_3)Fe]_2(μ-S)}^2- species have been isolated via chem. redn. and structurally characterized. Addnl. reactivity of these complexes has been investigated, including the reaction of [(PhBP_3)Fe]_2(μ-S) with CO to yield [(PhBP_3)FeCO]_2 (μ-S). This complex also shows a reversible redn. and evidence of reductive electrocatalytic activity under acidic conditions has been obsd.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47764, title ="Trigonal-bipyramidal, divalent Group 10 metal complexes supported by a tris(phosphino)silyl ligand", author = "Tsay, Charlene and Mankad, Neal P.", journal = "Abstracts of Papers of the American Chemical Society", volume = "241", pages = "INOR 939", month = "March", year = "2011", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140801-082324981", note = "© 2011 American Chemical Society.", revision_no = "11", abstract = "The mono-anionic, tetradentate tris(phosphino)silyl ligand, [SiP_R^3]- = [(2-R_2PC_6H_4)_3Si]-, provides access to\nelectrophilic, trigonal bipyramidal Group 10 cations {[SiP_R^3]M(L)}+ that feature weakly coordinated ligands\nincluding toluene, dihydrogen, and dinitrogen. The toluene adduct {[SiP^(Ph)_3]Pt(tol)}+ shows a close contact between the metal center and an aryl C-H bond in the solid state. For the more electron-rich isopropylsubstituted ligand, it is possible to exclude the fifth, axial ligand to afford the four-coordinate, trigonal pyramidal complexes {[SiP^(iPr)_3]Pt}+ and {[SiP^(iPr)_3]Pd}+, in contrast to prototypical d^8 square-planar Pt and Pd geometries. The corresponding cationic Ni complexes reversibly coordinate dinitrogen and dihydrogen.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/23163, title ="Terminal Iron Dinitrogen and Iron Imide Complexes Supported by a Tris(phosphino)borane Ligand", author = "Moret, Marc-Etienne and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "50", number = "9", pages = "2063-2067", month = "February", year = "2011", doi = "10.1002/anie.201006918", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110329-153028868", note = "© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.\nReceived: November 4, 2010.\nArticle first published online: 26 January 2011.\n\nThis work was supported by the NIH (GM 070757). M.-E.M. acknowledges a Fellowship for Prospective Researchers from the Swiss National Science Foundation. We thank Charlene Tsay and Larry Henling for crystallographic assistance, as well as David VanderVelde for assistance with DOSY experiments and Angelo Di Bilio for EPR measurements.", revision_no = "25", abstract = "Adaptable metallaboratranes: A tris- (phosphino)borane ligand stabilizes both low-valent Fe-N_2 complexes and a mid-valent imido species with a Fe≡NAr bond, thanks to its ability to shuttle between trigonal-bipyramidal and pseudotetrahedral geometries by elongation of the apical iron–boron bond (see picture)", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/20638, title ="Four-Coordinate, Trigonal Pyramidal Pt(II) and Pd(II) Complexes", author = "Tsay, Charlene and Mankad, Neal P.", journal = "Journal of the American Chemical Society", volume = "132", number = "40", pages = "13975-13977", month = "October", year = "2010", doi = "10.1021/ja105284p", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20101102-101005798", note = "© 2010 American Chemical Society. Published In Issue \nOctober 13, 2010. Article ASAP September 21, 2010. Received: June 16, 2010. This work was generously supported by the NSF (CHE-0750234). We are grateful to Paul F. Oblad for conducting preliminary synthetic studies and to Dr. Peter Müller and Dr. David VanderVelde for assistance with XRD and NMR studies, respectively. \n", revision_no = "22", abstract = "We report herein the characterization of electrophilic,\ntrigonal bipyramidal {[SiP_3^R]Pt(L)}+ cations ([SiP_3\n^R] ) [(2-R_2PC_6H_4)_3Si]; R ) Ph, ^iPr) that feature weakly coordinated ligands including CH_2Cl_2, Et_2O, toluene, and H_2. A cationic toluene adduct that shows a close platinum aryl C-H σ-contact is perhaps most noteworthy in this context. For the isopropyl-substituted ligand, [SiP_3^(iPr)], it has proven possible to exclude the fifth axial donor to afford the rigorously four-coordinate, trigonal pyramidal (TP) complex {[SiP_3^(iPr)]Pt}^+. An isostructural TP palladium complex {[SiP_3^(iPr)]Pd}^+ is also accessible. Prototypical four-coordinate d^8 platinum and palladium complexes are square planar. The TP d^8 cations described herein are hence geometrically distinct.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/19597, title ="Ligand design for site-selective installation of Pd and Pt centers to generate homo- and heteropolymetallic motifs", author = "Suess, Daniel L. M. and Peters, Jonas C.", journal = "Chemical Communications", volume = "46", number = "35", pages = "6554-6556", month = "September", year = "2010", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100823-131457715", note = "© 2010 Royal Society of Chemistry.\n\nReceived 15th February 2010, Accepted 18th June 2010.\n\nThis work was funded in part by the NSF (CHE-0750234).\nDLMS is grateful for funding from the NSF-GRFP and the\nMIT Presidential Fellowship Program. Dr Peter Mu¨ ller is\nacknowledged for insightful discussions.", revision_no = "16", abstract = "The modular synthesis of a series of nitrogen-rich polydentate ligands that feature a common pincer-type framework is reported. These ligands allow for site-selective installation of palladium and platinum to give rise to bi- and trimetallic complexes that have d^(8)–d^(8) interactions. \n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47725, title ="Access to well-defined Ru(I) and Os(I) metalloradicals", author = "Takaoka, Ayumi and Gerber, Laura C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "240", pages = "INOR 024", month = "August", year = "2010", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140731-141514646", note = "© 2010 American Chemical Society.", revision_no = "12", abstract = "One electron redn. of Ru(II) and Os(II) complexes, [SiP^(iPr)_3]MCl (M = Ru, Os) [SiP^R_3]- ([SiP^R_3]- = (2-R_2PC_6H_4)_3Si-, R = iPr), under dinitrogen atm. yields the first well-defined and thoroughly characterized examples of mononuclear Ru(I) and Os(I) complexes, [SiP^(iPr)_3]M(N_2) (M = Ru, Os). EPR and DFT studies confirm their resp. metalloradical character, and reactivity studies expose both 1-electron and 2-electron redox transformations. In particular, reactions of [SiP^(iPr)_3]M(N_2) (M = Ru, Os) with an\norgnanic azide afford the unusual imido/nitrene complexes, [SiP^(iPr)_3]M(NAr) (M = Ru, Os; Ar = C_6H_4CF_3), which possess significant radical character on the 'ArN' moiety.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47726, title ="Exploring the feasibility of N_2 fixation at single and multiple iron sites", author = "Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "240", pages = "INOR 370", month = "August", year = "2010", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140731-142309739", note = "© 2010 American Chemical Society.", revision_no = "10", abstract = "Of central interest to hypotheses concerning iron's role in biol. nitrogen fixation are the accessible geometries\nand oxidn. states of proposed iron intermediates. Indeed, a key longstanding goal of coordination chem. is to\nmap the types of metal complexes that can be anticipated to have reasonable stability and to elucidate their\nreactivity patterns. Such information drives to the heart of mechanistic postulates for transformations that\noccur in metalloenzymes as complex as nitrogenase. Our group has an ongoing interest in exploring and\nhopefully expanding the boundaries of the types Fe-N_xH_y species that can be generated and characterized.\nThese types of species find broader context with respect to proposed pathways for biol. nitrogen fixation. My\nlecture will specifically focus on results from our lab pertaining to the types of Fe-N_xH_y complexes that are\naccessible for 4- and 5-coordinate geometries. In particular, I will discuss what we now know about some\nrather unusual formal oxidn. states and spin states that are synthetically available for low-coordinate iron\nfeaturing N_2 or N_2-related ligands, ranging from Fe(I) to Fe(IV), and whether such species motivate new\nhypotheses for the mode nitrogen fixation at the cofactor.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/19301, title ="E-Type Delayed Fluorescence of a Phosphine-Supported Cu_2(μ-NAr_2)_2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs", author = "Deaton, Joseph C. and Switalski, Steven C.", journal = "Journal of the American Chemical Society", volume = "132", number = "27", pages = "9499-9508", month = "July", year = "2010", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100805-140123621", note = "© 2010 American Chemical Society.\nReceived January 18, 2010.\nPublication Date (Web): June 17, 2010.\n\nThe authors thank the National Science\nFoundation for support through Grant CHE 0616782 under the\nGOALI Program (Grant Opportunity for Academic Liaison with\nIndustry). We thank the following people for technical assistance\nand insightful discussions: Ching W. Tang, Kevin Klubek, Shouquan\nHuo, Viktor Jarikov, Christopher Brown, Michael Landry,\nDebra Blondell, Shawn Reuter, and Dustin Comfort. We are grateful\nto Prof. Richard Eisenberg, University of Rochester, who was\ninstrumental in bringing about our collaboration.", revision_no = "18", abstract = "A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S_1−T_1 energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.\n", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/19353, title ="Triggering N_2 uptake via redox-induced expulsion of coordinated NH_3 and N_2 silylation at trigonal bipyramidal iron", author = "Lee, Yunho and Mankad, Neal P.", journal = "Nature Chemistry", volume = "2", number = "7", pages = "558-565", month = "July", year = "2010", issn = "1755-4330", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100810-073448823", note = "© 2010 Macmillan Publishers Limited.\n\nReceived 23 November 2009. \nAccepted 29 March 2010.\nPublished online 16 May 2010.\n\nWe acknowledge the National Institutes of Health (GM-070757). Funding for the\nMassechussetts Institute of Technology Department of Chemistry Instrumentation Facility\nwas provided in part by the National Science Foundation (NSF) (CHE-0234877). P. Mueller\nprovided assistance with XRD analyses. N.P.M. received an NSF graduate fellowship. We\nthank R.H. Holm and T.A. Betley at Harvard University for providing us with access to a\nMössbauer spectrometer.\nAuthor contributions: \nY.L., N.P.M. and J.C.P. conceived and designed the experiments, Y.L. and N.P.M.\nperformed the experiments and Y.L. and J.C.P. co-wrote the paper.", revision_no = "31", abstract = "The biological reduction of N_2 to give NH_3 may occur by one of two predominant pathways in which nitrogenous N_xH_y\nintermediates, including hydrazine (N_2H_4), diazene (N_2H_2), nitride (N^(3-)) and imide (NH^(2-)), may be involved. To test the\nvalidity of hypotheses on iron’s direct role in the stepwise reduction of N_2, model systems for iron are needed. Such\nsystems can test the chemical compatibility of iron with various proposed N_xH_y intermediates and the reactivity patterns\nof such species. Here we describe a trigonal bipyramidal Si(o-C_6H_4PR_2)_3Fe–L scaffold (R=Ph or i-Pr) in which the apical\nsite is occupied by nitrogenous ligands such as N_2, N_2H_4, NH_3 and N_2R. The system accommodates terminally bound N_2\nin the three formal oxidation states (iron(0), +1 and +2). N_2 uptake is demonstrated by the displacement of its reduction\npartners NH_3 and N_2H_4, and N_2 functionalizaton is illustrated by electrophilic silylation.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44651, title ="Efficient luminescence from easily prepared three-coordinate copper(I) arylamidophosphines", author = "Lotito, Kenneth J. and Peters, Jonas C.", journal = "Chemical Communications", volume = "46", number = "21", pages = "3690-3692", month = "June", year = "2010", doi = "10.1039/C000818D", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140403-160228002", note = "This journal is © The Royal Society of Chemistry 2010. \n\nReceived 13th January 2010, Accepted 30th March 2010, First published on the web 14th April 2010. \n\nThis work was supported by an NSF Center for Chemical Innovation (CHE-0802907). Luminescence measurements were carried out in the laboratory of T. M. Swager with assistance from T. L. Andrews. K. J. L. gratefully acknowledges a fellowship from the MIT Energy Initiative.\n\nElectronic supplementary information (ESI) available: Synthetic procedures and characterization, absorption spectrum of 6, excitation and emission spectra, electrochemical data, and solid state structures. CCDC 766167 (1), 766168 (2), 766169 (3), 766170 (7). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c000818d", revision_no = "13", abstract = "A series of brightly luminescent, three-coordinate copper(I) arylamidophosphine complexes have been prepared from readily available precursors in high yield. Emission maxima span 102 nm in the visible spectrum from 461 (blue) to 563 nm (yellow) while photoluminescence quantum yields range from 0.11 to 0.24 in fluid solution at room temperature.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/18957, title ="Access to Well-Defined Ruthenium(I) and Osmium(I) Metalloradicals", author = "Takaoka, Ayumi and Gerber, Laura C. H.", journal = "Angewandte Chemie International Edition", volume = "49", number = "24", pages = "4088-4091", month = "June", year = "2010", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20100709-101044545", note = "© 2010 Wiley.\n\nReceived: 26 February 2010; Published online: May 7, 2010.\n\nThis work was supported by the NIH (GM070757). Samantha\nMacMillan and Dr. Peter Mller are acknowledged for crystallographic\nassistance. The NSF is acknowledged for use of instruments\nat the MIT DCIF (CHE-9808061, DBI-97299592).", revision_no = "15", abstract = "Radically complex: Well-defined mononuclear RuI and OsI complexes (see scheme) have metalloradical character, as indicated by EPR spectroscopy and DFT calculations. The Ru^I and Os^I metalloradicals exhibit both one-electron and two-electron redox reactivity. The latter process affords unusual imido complexes with substantial radical character on the {ArN} moiety.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52505, title ="Catalytic N−N Coupling of Aryl Azides To Yield Azoarenes via Trigonal Bipyramid Iron−Nitrene Intermediates", author = "Mankad, Neal P. and Müller, Peter", journal = "Journal of the American Chemical Society", volume = "132", number = "12", pages = "4083-4085", month = "March", year = "2010", doi = "ja910224c", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141209-131428584", note = "© 2010 American Chemical Society\n\nReceived December 3, 2009.\nPublication Date (Web): March 3, 2010.\n\nThis work was funded by the NIH (GM-070757). Tim Kowalczyk and Prof. Seth Brown provided helpful\ndiscussions.", revision_no = "12", abstract = "The reactivity of the trigonal bipyramidal iron(I) complex [SiP^(iPr)_3]Fe(N_2) ([SiP^(iPr)_3] = (2-iPr_2PC_6H_4)_3Si−) toward organoazides has been examined. 1-Adamantylazide was found to coordinate the iron center to form stable [SiP^(iPr)_3]Fe(η^1-N_3Ad). Aryl azides instead afforded unstable [SiP^(iPr)_3]Fe(N_3Ar) species that decayed gradually to regenerate [SiP^(iPr)_3]Fe(N_2) with release of azoarenes (ArN═NAr). The conversion of aryl azides to azoarenes can thus be achieved catalytically. Competitive trapping experiments strongly suggest the intermediacy of reactive nitrene complexes of the type [SiP^(iPr)_3]Fe(NAr) that couple bimolecularly in the N−N bond forming step. Evidence for one such intermediate was provided by electron paramagnetic resonance spectroscopy via photolysis of [SiP^(iPr)_3]Fe(N_3Ar) in a frozen glass. The electronic structures of these putative nitrene intermediates have been examined by DFT methods.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44647, title ="Hydrogen evolution by cobalt tetraimine catalysts adsorbed on electrode surfaces", author = "Berben, Louise A. and Peters, Jonas C.", journal = "Chemical Communications", volume = "46", number = "3", pages = "398-400", month = "January", year = "2010", doi = "10.1039/B921559J", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140403-153248105", note = "This journal is © The Royal Society of Chemistry 2010. \n\nReceived (in Berkeley, CA, USA) 14th October 2009, Accepted 10th November 2009, First published on the web 25th November 2009. \n\nThis work was supported by an NSF Center for Chemical Innovation (CHE-0802907) and by the Chesonis Family Foundation. Grants from NSF provided instrument support to the DCIF at MIT (CHE-9808061, DBI-9729592). We thank Dr Bruce Parkinson for helpful comments. \n\nElectronic supplementary information (ESI) available: Syntheses and characterization of compounds, plots of electrochemical measurements, CIF files for solid-state structures. CCDC 735555–735559. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b921559j", revision_no = "11", abstract = "Aryl-substituted tetraimine complexes related to Co(dmgBF_2)_2(MeCN)_2 (dmg = dimethylglyoxime) were synthesized and are active for hydrogen evolution. Co(dmgBF_2)_2(MeCN)_2 can be adsorbed to a glassy carbon electrode. The chemically modified electrode is active for hydrogen evolution in aqueous solution at pH < 4.5, with an overpotential of only 100 mV.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44646, title ="Redox rich dicobalt macrocycles as templates for multi-electron transformations", author = "Szymczak, Nathaniel K. and Berben, Louise A.", journal = "Chemical Communications", volume = "2009", number = "44", pages = "6729-6731", month = "November", year = "2009", doi = "10.1039/B913946J", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140403-151953405", note = "This journal is © The Royal Society of Chemistry 2009. \n\nReceived 13 Jul 2009, Accepted 17 Sep 2009, First published online 06 Oct 2009. \n\nThis work was generously supported by BP and an NSF Center for Chemical Innovation grant (Grant CHE-0802907). We thank Dr Yunho Lee and Dr Sebastian Stoian for EPR assistance, and Dr Bruce Brunschwig for helpful discussions. \n\nElectronic supplementary information (ESI) available: Complete experimental details and crystal structures. CCDC 740044–740052. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b913946j", revision_no = "11", abstract = "Pyridazine-templated dicobalt macrocycles reversibly support five oxidation states with unusually positive Co^(II)/Co^I redox couples, and are also active proton reduction electrocatalysts.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52510, title ="Characterization of Structurally Unusual Diiron N_xH_y Complexes", author = "Saouma, Caroline T. and Müller, Peter", journal = "Journal of the American Chemical Society", volume = "131", number = "30", pages = "10358-10359", month = "August", year = "2009", doi = "10.1021/ja903967z", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141209-142810433", note = "© 2009 American Chemical Society.\n\nPublication Date (Web): July 14, 2009.\n\nWe acknowledge the NIH (GM-070757). Funding for the MIT Department of Chemistry Instrumentation Facility has been provided in part by the NSF (CHE-0234877). C.T.S. is grateful for an NSF graduate fellowship.", revision_no = "15", abstract = "Mechanistic proposals concerning the pathway of N_2 reduction in biology at the MoFe-cofactor of nitrogenase continue to be advanced. In addition to nitrogen, hydrazine2 and diazene1a are nitrogenase substrates, and recent DFT calculations and spectroscopic studies suggest that whereas initial N_2 binding may occur at one iron center, diiron pathways may be involved at certain N_xH_y intermediate stages en route to ammonia formation. In this broad context, recent work has explored the synthesis and spectroscopic characterization of structurally unusual mono- and bimetallic iron complexes featuring nitrogenous ligand functionalities. The demand for such model complexes continues in light of recent ENDOR and ESEEM spectroscopic data that has been obtained under turnover conditions at the cofactor. To date, there are few synthetic iron systems that feature parent hydrazine (N_2H_4), hydrazido (N_2H_2^(2-)), diazene (N_2H_2), amide (NH_(2)^-), and imide (NH^2-) ligands. Herein we describe the synthesis and characterization of a series of structurally distinct diiron complexes that feature each of these ligand types.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/15534, title ="Multifrequency EPR Studies of [Cu^(1.5)Cu^(1.5)]+ for Cu_2(μ-NR_2)_2 and Cu_2(μ-PR_2)_2 Diamond Cores", author = "Mankad, Neal P. and Harkins, Seth B.", journal = "Inorganic Chemistry", volume = "48", number = "15", pages = "7026-7032", month = "August", year = "2009", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090901-145018265", note = "Copyright © 2009 American Chemical Society. \n\nReceived September 30, 2008. Publication Date (Web): July 2, 2009. \n\nJ.C.P. acknowledges support from the NSF (GOALI). N.P.M. is grateful for an NSF graduate fellowship. W.E.A. acknowledges the National Biomedical EPR Center Grant EB001980 from NIH. The authors are grateful to a reviewer concerning the EPR simulations. \n\nSupporting Information: Crystallographic data (CIF) and supporting EPR spectra are available free of charge via the Internet at http://pubs.acs.org.", revision_no = "19", abstract = "Multifrequency electron paramagnetic resonance (EPR) spectroscopy is used to explore the electronic structures of a series of dicopper complexes of the type {(LXL)Cu}_2^+. These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two [LXL]^− ligands featuring bridging amido or phosphido ligands and associated thioether or phosphine chelate donors. Specific chelating [LXL]^− ligands examined in this study include bis(2-tert-butylsulfanylphenyl)amide (SNS), bis(2-di-iso-butylphosphinophenyl)amide (PNP), and bis(2-di-iso-propylphosphinophenyl)phosphide (PPP). To better map the electronic coupling to copper, nitrogen, and phosphorus in these complexes, X-, S-, and Q-band EPR spectra have been obtained for each complex. The resulting EPR parameters implied by computer simulation are unusual for typical dicopper complexes and are largely consistent with previously published X-ray absorption spectroscopy and density functional theory data, where a highly covalent {Cu_2(μ-XR_2)_2}^+ diamond core has been assigned in which removal of an electron from the neutral {Cu_2(μ-XR_2)_2} can be viewed as ligand-centered to a substantial degree. To our knowledge, this is the first family of dicopper diamond core model complexes for which the compendium of X-, S-, and Q-band EPR spectra have been collected for comparison to Cu_A.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/15525, title ="E−H Bond Activation Reactions (E = H, C, Si, Ge) at Ruthenium: Terminal Phosphides, Silylenes, and Germylenes", author = "Takaoka, Ayumi and Mendiratta, Arjun", journal = "Organometallics", volume = "28", number = "13", pages = "3744-3753", month = "July", year = "2009", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20090901-115356130", note = "Copyright © 2009 American Chemical Society. \n\nReceived March 20, 2009. Publication Date (Web): June 16, 2009. \n\nThis work was supported by the NSF (CHE-0750234). Dr. Jeffrey H. Simpson is acknowledged for assistance with NMR studies. Dr. Peter Müller, Neal Mankad, and Samantha MacMillan are acknowledged for crystallographic assistance, and Paul Oblad is acknowledged for a preliminary reaction study. The NSF is acknowledged for its support of the DCIF at MIT (CHE-9808061 and DBI-9729592). We also acknowledge a reviewer for a helpful suggestion regarding the η3 formulation for complex 4. \n\nSupporting Information: X-ray crystallographic data, kinetic data, and NMR spectra for 4, 5, 8, 9, 10b, 12, and 13. This material is available free of charge via the Internet at http://pubs.acs.org.", revision_no = "22", abstract = "The placement of a strongly trans-influencing ligand on a ruthenium center opposite an anchoring silyl group of the tetradentate tripodal tris(phosphino)silyl ligand, [SiP^(Ph)_3]^− ([SiP^(Ph)_3]^− = tris(2-(diphenylphosphino)phenyl)silyl), has been explored. Installation of alkyl or terminal phosphide ligands trans to the anchoring silyl group affords the complexes [SiPPh3]RuR (R = Me (2), CH_2Ph (4), PPh_2 (5), P^iPr_2 (6)). Complexes 2, 4, and 5 are thermally unstable. Complexes 2 and 4 decay to the cyclometalated complex [SiP^(Ph)_2P′^(Ph)]Ru (3), whereas complex 5 decays to the cyclometalated phosphine adduct [SiP^(Ph)_2P′^(Ph)]Ru(PHPh_2) (7). Complex 3 is found to effect E−H (E = H, C, Si, Ge) bond activation of substrates such as secondary silanes and germanes to yield the structurally unusual silylene complexes [SiP^(Ph)_3]Ru(H)(SiRR′) (R = R′ = Ph (10a), R = Ph R′ = Me (10b)) and the germylene complex [SiP^(Ph)_3]Ru(H)(GeR_2) (R = Ph) (11) via double E−H activation transformations. Both theory and experiments suggest electrophilic character at the silylene moiety. Reaction of 3 with catecholborane, in contrast to silanes and germanes, results in insertion of the B−H unit into the M−C bond of the cyclometalated species to yield the borate complex [SiP^(Ph)_2P^(Ph)-B(cat)]Ru(μ-H) (14). Complex 3 also reacts with bis(catecholato)diboron to yield a similar complex, [SiP^(Ph)_2P^(C6H3B(cat))-B(cat)]Ru(μ-H) (15), with selective borylation of an ortho C−H bond.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52508, title ="Three-Coordinate Copper(I) Amido and Aminyl Radical Complexes", author = "Mankad, Neal P. and Antholine, William E.", journal = "Journal of the American Chemical Society", volume = "131", number = "11", pages = "3878-3880", month = "March", year = "2009", doi = "10.1021/ja809834k", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141209-142543909", note = "© 2009 American Chemical Society.\n\nReceived December 18, 2008.\n\nThis work was generously supported by the NSF (CHE-0750234; J.C.P.), the ONR (N00014-06-1016; R.K.S.), and the National Biomedical ESR Center Grant EB001980 from the NIH (W.E.A.). N.P.M. is grateful for an NSF graduate research fellowship. Portions of this research were carried out at the Stanford\nSynchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. DOE. The SSRL Structural Molecular Biology Program is supported by the DOE, Office of Biological and Environmental Research, and by the NIH, National Center for Research Resources, Biomedical Technology Program.", revision_no = "12", abstract = "Electron transfer (ET) through proteins often utilizes copper-containing active sites as efficient one-electron relays. The type-1 active sites of the blue copper proteins are prominent examples. It is generally thought that high ET rates through type-1 redox sites occur because the protein environments enforce unusual trigonally distorted coordination spheres to allow for minimal structural\nreorganization during ET. Though large Cu^II/Cu^I self-exchange ET rate constants (kS) in the range observed for type-1 sites have been achieved in certain synthetic monocopper systems using geometries distinct from trigonal environments, ET studies have yet to be conducted in a synthetic system featuring isolated, trigonally disposed copper centers. The simplest such systems would contain a trigonal planar geometry.\nHere we report the structural characterization of a trigonal planar system featuring formally Cu^II and Cu^I amido complexes related by a reversible one-electron redox event. We find in this system that ET is extremely rapid and is accompanied by a small degree of structural reorganization during redox. We propose that this structural rigidity in the absence of secondary coordination sphere effects results from significant covalency of the copper-amide linkages. In fact, a CuI-aminyl radical description of the formally Cu^II-amide complex may be most appropriate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52522, title ="Five-coordinate iron(II) NXHY complexes supported by tris(phosphino)silyl ligands: Oxidation and reduction of iron bound hydrazine", author = "Lee, Yunho and Mankad, Neal P.", journal = "Abstracts of Papers of the American Chemical Society", volume = "237", pages = "INOR-394", month = "March", year = "2009", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-081309994", note = "© 2009 American Chemical Society.", revision_no = "10", abstract = "Mapping the pathway of N_2 redn. by nitrogenase enzymes remains a coveted goal for synthetic chemists. To aid in elucidating\nthe pathways of biol. nitrogen fixation, studies of the coordination behavior of various reduced N_2 ligands bound to a single\niron site are of interest. Recently we began to work with anionic SiP^R_3 ligands (R = Ph and iPr) in order to accommodate\nmononuclear 5-coordinate iron species that exhibit paramagnetic ground states in both the iron(I) and iron(II) oxidn. states.\nDetails of various iron complexes with nitrogenous ligands, such as 5-coordinate iron(II) dinitrogen, hydrazine, ammonia,\nhydrazido and diazenido species, will be presented in addn. to reactivity studies including base catalyzed N-N bond cleavage,\noxidn. and hydrogenation chem.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52525, title ="The quest for earth abundant hydrogen evolution catalysts", author = "Peters, Jonas C. and Berben, Louise A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "237", pages = "INOR-635", month = "March", year = "2009", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-082353247", note = "© 2009 American Chemical Society.", revision_no = "10", abstract = "Our group has been studying late metal coordination complexes to explore factors that might lead to efficient\nelectrocatalysts for hydrogen evolution using comparatively earth abundant metals. Some of the\ncatalysts we have studies mediate hydrogen evolution at strikingly low overpotentials. This talk will\ndescribe our most recent efforts, including new mono- and dinuclear systems that display hydrogenase function.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52524, title ="Transition metal complexes with unusual geometries and/or oxidation states", author = "Peters, Jonas C. and Mankad, Neal P.", journal = "Abstracts of Papers of the American Chemical Society", volume = "237", pages = "INOR-254", month = "March", year = "2009", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-082033021", note = "© 2009 American Chemical Society.", revision_no = "11", abstract = "Transition metal complexes that push the boundaries of current dogma with regard to geometry and\nelectronic structure continue to be of fundamental interest. Our group has been exploring coordination\ncomplexes supported by tris(phosphino)borate, and recently also tris(phosphino)silyl ligands. This talk will\ntouch on our most recent efforts to prep. interesting complexes using these ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52521, title ="Understanding iron-catalyzed alkene hydrogenation: Unraveling the role of spin-state changes", author = "Besora, Maria and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "237", pages = "INOR-282", month = "March", year = "2009", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-080534042", note = "© 2009 American Chemical Society.", revision_no = "12", abstract = "Organometallic catalysts based on cheaper first-row transition metals are increasingly popular. Recently one of us described a\nnew set of complexes [PhBP3]Fe-R, R = Me, Bn, CH2CMe3, based on a mono-anionic trisphosphine ligand [PhBP3]. These\nspecies catalyze hydrogenation of alkenes and alkynes. Sep., the complexes have also been shown to react slowly with\ndihydrogen in the presence of a phosphine to provide an Fe(IV) trihydride species [PhBP3]FeH3L. The starting Fe(II) species is\nparamagnetic, whereas the trihydride is diamagnetic. In this talk, we will present work aimed at elucidating the mechanism of\nthe dihydrogen addn. reaction as well as the hydrogenation, in a bid to develop improved catalysts. It will be shown that all of\nthe key reactions occur over low-spin singlet (or in some cases triplet) transition states, whereas the key intermediates, [PhBP3]\nFe-R and [PhBP3]Fe-H, have high-spin quintet ground states. The computational methodol. used to (a) accurately describe the\nrelative energetics of the high- and low-spin states, and (b) characterize the rates of the spin-change events will be described.\nThe key mechanistic conclusion is that the hydrogenation catalytic cycle involves repeated changes from the less reactive but\nmore stable high-spin [Fe]-H and [Fe]-R species to lower spin adducts with H2 or alkene. This has the effect of decreasing\nreactivity compared to a hypothetical analogous low-spin iron catalyst. However, the latter low-spin catalyst would probably\nalso be inhibited by addn. of extraneous ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52511, title ="Dinitrogen Complexes Supported by Tris(phosphino)silyl Ligands", author = "Whited, Matthew T. and Mankad, Neal P.", journal = "Inorganic Chemistry", volume = "48", number = "6", pages = "2507-2517", month = "March", year = "2009", doi = "10.1021/ic801855y", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141209-142846211", note = "© 2009 American Chemical Society.\n\nReceived September 29, 2008.\n\nWe acknowledge the NIH (GM-070757), the Moore Foundation (fellowship to M.T.W.), and the National Science Foundation (fellowship to N.P.M.). Larry Henling provided crystallographic assistance.", revision_no = "12", abstract = "The tetradentate tris(phosphino)silyl ligand [Si^P(iPr)_3] ([SiP^(iPr)_3] = [Si(o-C_6H_4P^(iPr)_2)_3]−) has been prepared, and its complexation with iron, cobalt, nickel, and iridium precursors has been explored. Several coordination complexes have been thoroughly characterized and are described. These include, for example, the divalent trigonal bipyramidal metal chlorides [SiP^(iPr)_3]M−Cl (M = Fe, Co, Ni), as well as the monovalent dinitrogen adducts [SiP^(iPr)_3]M−N_2 (M = Fe, Co, Ir), which are compared with related [SiP^(Ph)_3]M−Cl and [SiP^(Ph)_3]M−N_2 species (M = Fe, Co). Complexes of this type represent the first examples of terminal dinitrogen adducts of monovalent iron, and the ligand architecture allows examination of a unique class of dinitrogen adducts with a trans-disposed silyl donor. Oxidation of the appropriate [SiP^(R)_3]M−N_2 precursors affords the divalent iron triflate [SiP^(Ph)_3]Fe(OTf) and trivalent cobalt triflate {[SiP^(iPr)_3]Co(OTf)}{OTf} complexes, which are of interest for group transfer studies because of the presence of a labile triflate ligand. Comparative electrochemical, structural, and spectroscopic data are provided for these complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/44655, title ="Dimanganese and Diiron Complexes of a Binucleating Cyclam Ligand: Four-Electron, Reversible Oxidation Chemistry at High Potentials", author = "Berben, Louise A. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "47", number = "24", pages = "11669-11679", month = "December", year = "2008", doi = "10.1021/ic801289x", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140403-192533870", note = "Copyright © 2008 American Chemical Society. \n\nPublished In Issue December 15, 2008. Article ASAP November 13, 2008. Received: July 11, 2008. \n\nWe acknowledge support from an NSF Chemical Bonding Center (Grant CHE-0533150). L.A.B. was partially supported by a Dow Chemical Co. postdoctoral fellowship from the American Australian Association.\n\nSupporting Information:\n\nCIF files for the structures of 4−6, EPR spectra of 4 in MeCN and MeCN/H2O (8:2). UV−vis and EPR spectra of the reaction solutions of 4 and 6 with PhIO, cyclic voltammetry data for 6 and 7, and plot of V(O2)/mL vs t/s for catalase measurements of 4. This material is available free of charge via the Internet at http://pubs.acs.org.", revision_no = "19", abstract = "The reaction of a binucleating biscyclam ligand cyclam_2^iPrO [where cyclam_2^iPrO = (1,3-bis[1,4,8,11-tetraazacyclododecane]-2-hydroxypropane] with Mn(CF_3SO_3)_2 or Fe(CF_3SO_3)_2•2MeCN gives [(cyclam_2^iPrO)Mn_2(μ-CF_3SO_3)](CF_3SO_3)_2 (4) and [(cyclam_2^iPrO)Fe_2(μ-CF_3SO_3)](CF_3SO_3)_2 (6), respectively. [(cyclam_2^iPrO)Mn_2(μ-N_3)](CF_3SO_3)_2 (5) is obtained by the reaction of 4 with NaN_3. Single-crystal X-ray structural characterization indicates that in each of the bimetallic complexes the two metal centers are facially coordinated by a cyclam ligand and bridged by the isopropoxide linker of the ligand in addition to a triflate counteranion. Upon replacement of the triflate bridge with the single-atom bridge of an end-bound azide ligand in 5, the Mn—Mn distance decreases by 0.38 Å. All of the complexes are high-spin and colorless and were characterized by magnetic susceptibility measurements, electron paramagnetic resonance spectroscopy, and electrochemical methods. Magnetic susceptibility measurements indicate that 4 and 6 are weakly antiferromagnetically coupled while 5 is weakly ferromagnetically coupled. Cyclic voltammetry measurements indicate that the hard donor amine ligands impart high oxidation potentials to the metal centers and that four-electron redox activity can be accessed with a narrow potential range of 0.72 V. Upon inclusion of water in the cyclic voltammetry experiment, the oxidative waves shift to higher potentials, which is consistent with water binding the manganese centers. The diiron complex 6 displays four one-electron redox couples, of which the final two are irreversible. Inclusion of water in the cyclic voltammetry measurement for compound 6 resulted in two sets of shifted peaks, which suggests that two molecules of water bind the diiron core. In accordance with the observed reversibility of the electrochemical results, the dimanganese complex is more efficient than the diiron complex for mediating O-atom transfer to organic substrates and is an excellent hydrogen peroxide disproportionation catalyst, with the reaction proceeding for over 20\u2009000 turnovers.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/12859, title ="Group VIII Coordination Chemistry of a Pincer-Type Bis(8-quinolinyl)amido Ligand", author = "Betley, Theodore A. and Qian, Baixin A.", journal = "Inorganic Chemistry", volume = "47", number = "24", pages = "11570-11582", month = "December", year = "2008", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:BETic08", note = "Copyright © 2008 American Chemical Society. \n\nReceived June 11, 2008. Publication Date (Web): November 13, 2008. \n\nJ.C.P. is grateful to the NSF for financial support of this work (CHE-0750234 and CHE-0132216). T.A.B. is grateful to the Department of Defense for a graduate research fellowship. \n\nSupporting Information: Crystallographic data (PDF and CIF). This material is available free of charge via the Internet at http://pubs.acs.org.", revision_no = "15", abstract = "This paper provides an entry point to the coordination chemistry of the group VIII chemistry of the bis(8-quinolinyl)amine (BQA) ligand. In this context, mono- and disubstituted BQA complexes of iron, ruthenium, and osmium are described. For example, the low-spin bis-ligated Fe(III) complex [Fe(BQA)2][BPh4] has been prepared via amine addition to FeCl3 in the presence of a base and NaBPh4. Complexes featuring a single BQA ligand are more readily prepared for Ru and Os. Auxiliary ligands featuring a single BQA ligand, along with two other L-type donor ligands, allow for a variety of ligand types to occupy a sixth coordination site. Representative examples include the halide and pseudohalide complexes trans-(BQA)MX(PPh3)2 (M = Ru, Os; X = Cl, Br, N3, OTf), as well as the hydride and alkyl complexes trans-(BQA)RuH(PMe3)2 and trans-(BQA)RuMe(PMe3)2. Electrochemical studies are discussed that help to contextualize the BQA ligand with respect to its neutral counterpart 2,2′,2′′-terpyridine (terpy) in terms of electron-releasing character. Bidentate ligands have been explored in conjunction with the BQA ligand. Thus, the bidentate, monoanionic aryl(8-quinolinyl)amido ligand 3,5-(CF3)2-(C6H3)QA has been installed onto the (BQA)Ru platform to provide (BQA)Ru(3,5-(CF3)2-(C6H3)QA)(PPh3). A bis(phosphino)borate ligand stabilizes the five-coordinate complex [Ph2B(CH2PPh2)2]Ru(BQA). Finally, access to dinitrogen complexes of the types [(BQA)Ru(N2)(PPh3)2][PF6], [(BQA)Ru(N2)(PMe3)2][PF6], and [(BQA)Os(N2)(PPh3)2][PF6] is provided by exposure of the sixth coordination site under a N2 atmosphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/12675, title ="Phosphido Pincer Complexes of Palladium as New Efficient Catalysts for Allylation of Aldehydes", author = "Mazzeo, Mina and Lamberti, Marina", journal = "Organometallics", volume = "27", number = "22", pages = "5741-5743", month = "November", year = "2008", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:MAZo08", note = "Copyright © 2008 American Chemical Society. \n\nReceived September 3, 2008. Publication Date (Web): October 22, 2008. \n\nWe thank Matt T. Whited for assistance with crystallographic studies. \n\nSupporting Information: Text, figures, and tables giving experimental procedures for the synthesis and characterization data of compounds 1 and 2 and X-ray structural solution and crystal data for 1; X-ray data for 1 are also given as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.", revision_no = "21", abstract = "Palladium complexes supported by tridentate phosphido diphosphine ligands (P(o-C6H4PR2)2: 1, R = iPr; 2, R = Ph) have been synthesized, characterized, and tested as catalysts for the electrophilic allylation of aldehydes. The palladium complex 2 resulted in an interesting catalyst for electrophilic allylation in the presence of allyltributyltin, giving good yields under very mild reaction conditions and even in the absence of the solvent.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47956, title ="Bis(α-diimine)iron Complexes: Electronic Structure Determination by Spectroscopy and Broken Symmetry Density Functional Theoretical Calculations", author = "Muresan, Nicoleta and Lu, Connie C.", journal = "Inorganic Chemistry", volume = "47", number = "11", pages = "4579-4590", month = "April", year = "2008", doi = "10.1021/ic7022693", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-072235251", note = "© 2008 American Chemical Society. Received November 19, 2007.\nPublication Date (Web): April 29, 2008. N.M. and M.G. are grateful to the Max-Planck Society for a postdoctoral stipend and C.C.L. thanks the A. v. Humboldt Foundation for a fellowship. We gratefully acknowledge financial support from the Fonds of the Chemical Industry. M.A. thanks the Caltech SURF program for funding. We also would like to thank Dr. K.\nChlopek for preliminary DFT calculations of 8.", revision_no = "15", abstract = "The electronic structure of a family comprising tetrahedral (α-diimine)iron dichloride, and tetrahedral bis(α-diimine)iron compounds has been investigated by Mössbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. In addition, broken-symmetry density functional theoretical (B3LYP) calculations have been performed. A detailed understanding of the electronic structure of these complexes has been obtained. A paramagnetic (S_t = 2), tetrahedral complex [Fe^(II)(^4L)_2], where (^4L)^(1−) represents the diamagnetic monoanion N-tert-butylquinolinylamide, has been synthesized and characterized to serve as a benchmark for a Werner-type complex containing a tetrahedral Fe^(II)N_4 geometry and a single high-spin ferrous ion. In contrast to the most commonly used description of the electronic structure of bis(α-diimine)iron(0) complexes as low-valent iron(0) species with two neutral α-diimine ligands, it is established here that they are, in fact, complexes containing two (α-diiminato)^(1−•) π radical monoanions and a high-spin ferrous ion (in tetrahedral N_4 geometry) (S_(Fe) = 2). Intramolecular antiferromagnetic coupling between the π radical ligands (S_(rad) = 1/2) and the ferrous ion (S_(Fe) = 2) yields the observed S_t = 1 ground state. The study confirms that α-diimines are redox noninnocent ligands with an energetically low-lying antibonding π^* lowest unoccupied molecular orbital which can accept one or two electrons from a transition metal ion. The (α-diimine)FeCl_2 complexes (St = 2) are shown to contain a neutral α-diimine ligand, a high spin ferrous ion, and two chloride ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47973, title ="Probing the Electronic Structures of [Cu_2(í-XR_2)]^(n+) Diamond Cores as a Function of the Bridging X Atom (X = N or P) and Charge (n = 0, 1, 2)", author = "Harkins, Seth B. and Mankad, Neal P.", journal = "Journal of the American Chemical Society", volume = "130", number = "11", pages = "3478-3485", month = "March", year = "2008", doi = "10.1021/ja076537v", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-113442019", note = "© 2008 American Chemical Society. Received September 1, 2007. Publication Date (Web): February 26, 2008. Financial support provided by the DOE (PECASE; J.C.P.); the ONR (N00014-06-1016; R.K.S.); and a NSF Graduate Research Fellowship (N.P.M.). Larry Henling and Dr. Mike Day provided crystallographic assistance. Portions of this research were carried out at the Stanford Synchrotron\nRadiation Laboratory, a national user facility operated by\nStanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program.", revision_no = "14", abstract = "A series of dicopper diamond core complexes that can be isolated in three different oxidation states ([Cu_2(μ-XR_2)]^(n+), where n = 0, 1, 2 and X = N or P) is described. Of particular interest is the relative degree of oxidation of the respective copper centers and the bridging XR_2 units, upon successive oxidations. These dicopper complexes feature terminal phosphine and either bridging amido or phosphido donors, and as such their metal−ligand bonds are highly covalent. Cu K-edge, Cu L-edge, and P K-edge spectroscopies, in combination with solid-state X-ray structures and DFT calculations, provides a complementary electronic structure picture for the entire set of complexes that tracks the involvement of a majority of ligand-based redox chemistry. The electronic structure picture that emerges for these inorganic dicopper diamond cores shares similarities with the Cu_2(μ-SR)_2 Cu_A sites of cytochrome c oxidases and nitrous oxide reductases.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52513, title ="Diazoalkanes react with a bis(phosphino)borate copper(I) source to generate [Ph_2BP^(tBu)_2]Cu(η^1-N_2CR_2), [Ph_2BP^(tBu)_2]Cu(CPh_2), and [Ph_2BP^(tBu)_2]Cu–N(CPh_2)(NCPh_2)", author = "Mankad, Neal P. and Peters, Jonas C.", journal = "Chemical Communications", volume = "2008", number = "9", pages = "1061-1063", month = "March", year = "2008", doi = "10.1039/b713687k ", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141209-143755965", note = "© 2008 Royal Society of Chemistry.\n\nReceived 06 Sep 2007, Accepted 14 Dec 2007,\nFirst published online 17 Jan 2008.\n\nThis work was funded by BP (MC2 program) and the NSF\n(CHE-0132216). N. P. M. is grateful for an NSF Graduate\nResearch Fellowship. J. Christopher Thomas is acknowledged\nfor preliminary reaction screening. John Keith gave useful\nadvice regarding DFT calculations, and Larry Henling provided\ncrystallographic assistance. DFT calculations were done\nat the Materials and Process Simulation Center at the California\nInstitute of Technology.", revision_no = "13", abstract = "[Ph_2BP^(tBu)_2]Cu–L complexes react with diazoalkanes to generate structurally unusual η^1-diazoalkane adducts, a terminal carbene, and an η^1-azine adduct.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48002, title ="X-ray photochemistry in iron complexes from Fe(0) to Fe(IV) – Can a bug become a feature?", author = "George, Simon J. and Fu, Juxia", journal = "Inorganica Chimica Acta", volume = "361", number = "4", pages = "1157-1165", month = "March", year = "2008", doi = "10.1016/j.ica.2007.10.039", issn = "0020-1693", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-140326162", note = "© 2007 Elsevier B.V. Received 19 October 2007; accepted 28 October 2007. Available online 4 November 2007. Dedicated to Edward Solomon. One of us (S.P.C.) would especially like to thank Ed Solomon for hosting him during ‘minisabbaticals’ at Stanford. The group meetings were always intellectually stimulating, and the frequent birthday cakes were a special treat. We thank the ALS staff, in particular Drs. Elke Arenholtz and Anthony Young for their support of beamline 4.0.2. This work was funded by NIH Grant EB-001962 (SPC). ABEX is supported by the DOE Office of Biological and Environmental Research. J.C.P. gratefully acknowledges support from the NSF (CHE-0132216), the DOE (PECASE), and the Alfred P. Sloan Foundation. T.B.R.\nthanks the NIH (GM-061153) for support. The Advanced Light Source is supported by the DOE Office of Science, Office of Basic Energy Sciences.", revision_no = "15", abstract = "Under intense soft X-ray irradiation, we have observed time-dependent changes in the soft X-ray spectra of virtually all the Fe coordination complexes that we have examined, indicating chemical transformation of the compound under study. Each compound, with oxidation states ranging from Fe(IV) to Fe(0), has been studied with either Fe L-edge spectroscopy or N K-edge spectroscopy. We find that very often a well-defined spectroscopic change occurs, at least initially, which is apparently capable of straightforward interpretation in terms of X-ray induced photoreduction, photooxidation or ligand photolysis. We briefly discuss the probable chemical nature of the changes and then estimate the rate of chemical change, thereby establishing the necessary radiation dose. We also demonstrate that the photochemistry not only depends on the Fe oxidation state but also the coordination chemistry of the complex. It seems that a proper understanding of such X-ray photochemical effects could well greatly assist the assignment of soft X-ray spectra of uncharacterized metal sites.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47670, title ="Long-Lived and Efficient Emission from Mononuclear Amidophosphine Complexes of Copper", author = "Miller, Alexander J. M. and Dempsey, Jillian L.", journal = "Inorganic Chemistry", volume = "46", number = "18", pages = "7244-7246", month = "September", year = "2007", doi = "10.1021/ic7009513", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-102723763", note = "© 2007 American Chemical Society\n\nReceived May 16, 2007.\nPublication Date (Web): August 3, 2007.\n\nWe acknowledge the NSF GOALI\nprogram. J.L.D. is grateful for an NSF GRF. Dr. Jay R.\nWinkler provided insightful insight. B.-L. Lin assisted with\ncalculations. Luminescence measurements were carried out\nat the Beckman Institute Laser Resource Center.", revision_no = "14", abstract = "A number of structurally unusual, monomeric amidophosphine complexes of copper exhibit luminescence properties that are unprecedented for monocopper systems in solution at room temperature. The complexes exhibit lifetimes as long as 150 μs in benzene and quantum efficiencies in the range of 0.16 < φ < 0.70.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47676, title ="Carbon dioxide activation by low-valent pseudo-tetrahedral iron", author = "Saouma, Caroline T. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "234", pages = "INOR 920", month = "August", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-132553802", note = "© 2007 American Chemical Society.", revision_no = "11", abstract = "Upon exposure to CO_2, pseudo-tetrahedral iron(I) supported by 1;PhBP^R_33;- ligands (where 1;PhBP^R_33; =1;PhB(CH_2PR2)_33;-, and R = CH_2Cy, ^iPr, and Ph) effects either the reductive cleavage of CO_2 via O-atom transfer or reductive coupling of CO_2 to give an oxalate. THF solns. of 1;PhBP^(CH2Cy)_33;Fe(I) gives rise to the structurally unprecedented- Fe_2(μ-O)(μ-CO), whereas 1;PhBP^(CH2Cy)_33;FePPh_3 gives a bridging oxalate species.\nIn situ sodium/amalgam redns. of 1;PhBP^R_33;FeCl under an atm. of CO_2 again gives Fe_2(μ-oxalate) for R =CH_2Cy, but gives a Fe_2O species as the major product for both R = -^iPr and Ph.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47678, title ="Catalytic hydrogen evolution and hydrogen oxidation by cobalt macrocyclic glyoxime and tetra-imine complexes", author = "Berben, Louise A. and Hu, Xile", journal = "Abstracts of Papers of the American Chemical Society", volume = "234", pages = "INOR 921", month = "August", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-133508654", note = "© 2007 American Chemical Society.", revision_no = "12", abstract = "Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF_2)_2(CH_3CN)_2 and Co(dpgBF_2)_2(CH_3CN_2) (where dmgBF_2 is difluoroboryldimethylglyoxime and dpgBF_2 is difluoroboryldiphenylglyoxime), as well as cobalt complexes with1;143;-tetraene-N_4(Tim) ligands of the type 1;Co(Tim^R)X_23;^n+ (R = Me or Ph, X = Br or CH_3CN; n = 1 with X = Br and n = 3 with X = CH_3CN) have been obsd. to evolve H_2 electrocatalytically at potentials between -0.55 V and -0.20 V vs. SCE in CH_3CN.\nThe potentials at which these cobalt complexes catalyzed H_2 evolution were close to the reported thermodn. potentials for the prodn. of H_2 from protons in CH_3CN, with the smallest overpotential being 40 mV for Co(dmgBF_2)_2(CH_3CN)_2. Consistent with these small overpotentials, some of the complexes were also able to oxidize H_2 in the presence of a suitable conjugate base.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47677, title ="Dinitrogen chemistry at five-coordinate iron centers", author = "Mankad, Neal P. and Whited, Matthew T.", journal = "Abstracts of Papers of the American Chemical Society", volume = "234", pages = "INOR 035", month = "August", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-133300030", note = "© 2007 American Chemical Society.", revision_no = "10", abstract = "Dinitrogen complexes of iron(I) are unusual and quite interesting in the context of a Chatt-type Fe(I)/Fe(IV) nitrogen fixation\nmechanism. Previous Fe(I)-N2 complexes with 3- or 4-coordinate Fe centers have been dinuclear with bridging dinitrogen\nligands. We will present the synthesis of new monoanionic tetradentate ligands with [SiP3] donor sets and discuss their use in\nprepg. 5-coordinate iron complexes, including the first terminal dinitrogen complexes of iron(I). The release of dinitrogen\nredn. products from these Fe(I)-N2 species under protolytic conditions, as well as synthetic efforts to access Fe-N multiple bonds\nin these new trigonal bipyramidal environments, will also be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47675, title ="Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes", author = "Hu, Xile and Brunschwig, Bruce S.", journal = "Journal of the American Chemical Society", volume = "129", number = "29", pages = "8988-8998", month = "July", year = "2007", doi = "10.1021/ja067876b", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-123328207", note = "© 2007 American Chemical Society. \n\nReceived November 3, 2006; Publication Date (Web): June 28, 2007. \n\nWe acknowledge support from an NSF Chemical Bonding Center (grant CHE-0533150) and from the Beckman Institute Molecular Materials Research Center. We thank Prof. Nathan S. Lewis, Prof. Harry B. Gray, and Dr. Jay Winkler for insightful discussions. We also thank Prof. Alex Sessions and Dr. Chao Li for their generous help with the gas chromatography measurements and Neal Mankad and Larry Henling for help with crystallographic studies.", revision_no = "17", abstract = "Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF_2)_2(CH_3CN)_2 and Co(dpgBF_2)_2(CH_3CN)_2 (where dmgBF_2 is difluoroboryl-dimethylglyoxime and dpgBF_2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N_4 (Tim) ligands of the type [Co(Tim^R)X_2]^(n+) (R = methyl or phenyl, X = Br or CH_3CN; n = 1 with X = Br and n = 3 with X = CH_3CN), have been observed to evolve H_2 electrocatalytically at potentials between −0.55 V and −0.20 V vs SCE in CH_3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H_2 production. For the complexes Co(dmgBF_2)_2(CH_3CN)_2, Co(dpgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br2]Br, and [Co(Tim^(Me))(CH_3CN)_2](BPh_4)_3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H_2 production. In contrast, the complexes [Co(Tim^(Ph/Me))Br_2]Br and [Co(Tim^(Ph/Me))(CH_3CN)_2](BPh_4)_3 were less stable, and bulk electrolysis only produced faradaic yields for H_2 production of 20−25%. Cyclic voltammetry of Co(dmgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br_2]^+, and [Co(Tim^(Me))(CH_3CN)_2]^(3+) in the presence of acid revealed redox waves consistent with the Co(III)−H/Co(II)−H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H_2 evolution were close to the reported thermodynamic potentials for the production of H_2 from protons in CH_3CN, with the smallest overpotential being 40 mV for Co(dmgBF_2)_2(CH_3CN)_2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF_2)_2(CH_3CN)_2 was also able to oxidize H_2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H_2 evolution catalysis, and these studies are discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47927, title ="Terminal Fe^I-N_2 and Fe^(II)···H-C Interactions Supported by Tris(phosphino)silyl Ligands", author = "Mankad, Neal P. and Whited, Matthew T.", journal = "Angewandte Chemie International Edition", volume = "46", number = "30", pages = "5768-5771", month = "July", year = "2007", doi = "10.1002/anie.200701188", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-145426707", note = "© 2007 Wiley-VCH Verlag GmbH & Co. Manuscript Received: 17 Mar 2007. Article first published online: 28 Jun 2007. This work was supported by the NIH (GM 070757) and BP (MC^2 program). N.P.M. was supported by an NSF Graduate Research Fellowship, and M.T.W. by a Moore Foundation Fellowship. Larry Henling and Dr. Angelo DiBilio are acknowledged for crystallographic assistance and EPR assistance, and Prof. John Bercaw assisted with a Toepler pump analysis.", revision_no = "14", abstract = "Taking hold: Monoanionic tris(phosphino)silyl ligands ([SiP6R-3]=[(2-R-2PC-6H-4)-3Si]−) stabilize unusual five-coordinate Fe^I complexes. The silane [SiP^(Ph)_3]H reacts with mesityliron(II) to give [(SiP^(Ph)_3)Fe^(II)Mes], which displays a C—H agostic interaction. Treatment with HCl and reduction affords the terminally bonded Fe^I—N_2 complex [(SiP^(Ph)_3)FeN_2] (see scheme; structures show only donor atoms of [SiP^(Ph)_3]), which yields hydrazine under protolytic conditions.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47966, title ="XAS Characterization of a Nitridoiron(IV) Complex with a Very Short Fe-N Bond", author = "Rohde, Jan-Uwe and Betley, Theodore A.", journal = "Inorganic Chemistry", volume = "46", number = "14", pages = "5720-5726", month = "June", year = "2007", doi = "10.1021/ic700818q", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-100721847", note = "© 2007 American Chemical Society. \n\nReceived April 28, 2007. Publication Date (Web): June 15, 2007. \n\nThis work was supported by NIH grants GM-33162 (L.Q.) and GM-070757 (J.C.P.) and postdoctoral fellowship FGM075700 (T.A.J.). We are also grateful for a Department of Defense Graduate Fellowship \nfor T.A.B. and a National Science Foundation for C.T.S. XAS data were collected on beamline 9-3 at the Stanford Synchrotron Radiation Laboratory (SSRL) and beamline X9B at the National Synchrotron Light Source (NSLS). The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of\nBiological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. We thank Dr. Matthew J. Latimer at SSRL and Dr. Nebojsa S. Marinkovic at NSLS for their excellent technical support of our synchrotron experiments, Prof. Michael P. Hendrich and Mr. William Gunderson of Carnegie Mellon University for obtaining a Mössbauer spectrum of sample 4C, and Professor Thomas Brunold of the University of Wisconsin-Madison for graciously providing access to his computer cluster.", revision_no = "22", abstract = "X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBP^R_3]Fe(N) complexes (R = iPr and CyCH_2) and obtain direct spectroscopic evidence for a very short Fe−N distance. The distance of 1.51−1.55 Å reflects the presence of an FeN triple bond in accord with the observed Fe_≡N vibration observed for one of these species (ν_(FeN)\u2009=\u20091034 cm^(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with Fe^I−N_2−Fe^I, Fe^(II)−NPh_2, and Fe^(III)_≡NAd motifs, and those of recently described six-coordinate Fe^V≡N and Fe^V≡IN complexes. The observation that the Fe^(IV)−N distances of two [PhBPR_3]Fe(N) complexes are shorter than the Fe^(IV)−O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater π basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47682, title ="35 GHz CW and pulsed ENDOR spectroscopy of a novel diiron imide-hydride complex", author = "Kinney, R. Adam and Saouma, Caroline", journal = "Abstracts of Papers of the American Chemical Society", volume = "233", pages = "INOR 1020", month = "March", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-151316761", note = "© 2007 American Chemical Society.", revision_no = "11", abstract = "A recently synthesized diiron imide-hydride complex L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3\nis of interest for its similarity to a nitrogenase intermediate suggested to contain a bridging hydride. Electron nuclear double resonance (ENDOR) studies on this intermediate show unprecedented signals from two chem. equiv. and symmetryrelated protons with large proton hyperfine couplings. To help det. the bonding characteristics of these hydrogenic species, a study of this model complex has been undertaken using 35 GHz CW and Pulsed ENDOR spectroscopy. CW ENDOR spectra show a ^1H signal with extremely large hyperfine couplings, A_(Max) = 60 MHz. The corresponding bridging-deuterated complex was prepd. and the ^2H ENDOR measurements\nwere performed by CW and pulsed ENDOR. DFT calcns. of the hyperfine values (ADF package) indicate that\nthe large coupling proton is due to the bridging hydride. ^1H, ^2H, and ^(14)N CW and Pulsed ENDOR spectra with simulations and ADF calcns. will be presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47681, title ="Electrocatalytic hydrogen evolution at positive potentials", author = "Peters, Jonas C. and Hu, Xile", journal = "Abstracts of Papers of the American Chemical Society", volume = "233", pages = "INOR 490", month = "March", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-151045636", note = "© 2007 American Chemical Society.", revision_no = "12", abstract = "Hydrogen prodn. using earth abundant metals is one of the important challenges facing an energy economy based upon solar\nfuels. Schemes that incorporate the photo-oxidn. of water must also include schemes for efficiently converting released protons\nand electrons into viable fuel sources such as hydrogen. In this talk mol. approaches to hydrogen prodn. will be discussed\nwith a particular emphasis on systems that function at mild overpotentials.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47679, title ="Small molecule activation chemistry with low valent iron", author = "Peters, Jonas C. and Mankad, Neal P.", journal = "Abstracts of Papers of the American Chemical Society", volume = "233", pages = "INOR 103", month = "March", year = "2007", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-144118300", note = "© 2007 American Chemical Society.", revision_no = "10", abstract = "Low valent iron complexes are rich in their ability to activate small mols. Our has been exploring a no. of\nphosphine-supported iron(I) complexes that show fascinating reaction chem. with small mol. substrates.\nAspects of this work will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47671, title ="Fe(I)-Mediated Reductive Cleavage and Coupling of CO_2:\u2009 An Fe^(II)(μ-O,μ-CO)Fe^(II) Core", author = "Lu, Connie C. and Saouma, Caroline T.", journal = "Journal of the American Chemical Society", volume = "129", number = "1", pages = "4-5", month = "January", year = "2007", doi = "10.1021/ja065524z ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-111633168", note = "© 2007 American Chemical Society.\n\nReceived July 31, 2006.\nPublication Date (Web): December 16, 2006.\n\nWe thank Larry M. Henling and Neal\nMankad for crystallographic assistance. C.T.S. is supported by an\nNSF graduate fellowship. We are grateful to the NIH (GM070757)\nand BP (MC2 program) for financial support.", revision_no = "17", abstract = "THF solutions of a new iron(I) source, [PhBP^(CH2_Cy_3)]Fe ([PhBP^(CH_2Cy_3)] = [PhBP(CH_2P(CH_2Cy)_2)_3]-), effect the reductive cleavage of CO_2 via O-atom transfer at ambient temperature. The dominant reaction pathway is bimetallic and leads to the formation of a structurally unprecedented diiron Fe^(II)(μ-O)(μ-CO)Fe^(II) core. X-ray data are also available to suggest that bimetallic reductive CO_2 coupling to generate oxalate occurs as a minor reaction pathway. These initial observations forecast a diverse reaction landscape between CO_2 and iron(I) synthons.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/11127, title ="On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV=N, and related species", author = "Hendrich, Michael P. and Gunderson, William", journal = "Proceedings of the National Academy of Sciences of the United States of America", volume = "103", number = "46", pages = "17107-17112", month = "November", year = "2006", doi = "10.1073/pnas.0604402103", issn = "0027-8424", url = "https://resolver.caltech.edu/CaltechAUTHORS:HENpnas06", note = "© 2006 by the National Academy of Sciences. \n\nEdited by Richard R. Schrock, Massachusetts Institute of Technology, Cambridge, MA, and approved August 29, 2006 (received for review May 30, 2006). Published online before print November 7, 2006. This article is a PNAS direct submission. \n\nSquid data were collected at the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology. This work was supported by National Institutes of Health Grant GM-070757 (to J.C.P.), Postdoctoral Fellowship GM-072291 (to M.P.M.), and Grant GM-077387 (to M.P.H.). R.K.B. is grateful for a Herman Frasch Foundation Fellowship, and M.T.G. acknowledges the Arnold and Mabel Beckman Foundation and the Alfred P. Sloan Foundation. \n\nAuthor contributions: M.P.H., M.T.G., and J.C.P. designed research; M.P.H., W.G., R.K.B., M.T.G., M.P.M., T.A.B., C.C.L., and J.C.P. performed research; M.P.H., W.G., R.K.B., M.T.G., M.P.M., and J.C.P. analyzed data; and M.P.H., M.P.M., and J.C.P. wrote the paper. \n\nThe authors declare no conflict of interest. \n\nComplete materials and methods are provided in Supporting Materials and Methods.", revision_no = "29", abstract = "The electronic properties of an unusually redox-rich iron system, [PhBPR 3]FeNx (where [PhBPR 3] is [PhB(CH2PR2)3]−), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBPiPr 3]FeN (3a) and [PhBPCH2Cy 3]FeN (3b), and the formally diiron(I) bridged-Fe(μ-N2)Fe species, {[PhBPiPr 3]Fe}2(μ-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of −0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBPiPr 3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBPiPr 3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBPiPr 3]FeCl and [PhBPiPr 3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47961, title ="Pseudotetrahedral Manganese Complexes Supported by the Anionic Tris(phosphino)borate Ligand [PhBP^(iPr)_3]", author = "Lu, Connie C. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "45", number = "21", pages = "8597-8607", month = "September", year = "2006", doi = "10.1021/ic060735q", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-091324053", note = "© 2006 American Chemical Society. Received April 29, 2006. Publication Date (Web): September 22, 2006. We gratefully acknowledge the NSF (CHE-0132216)) and the NIH (GM 070757) for financial support; the Beckman Institute (Caltech) for use of its crystallographic facility and SQUID magnetometer; the Materials and Process Simulation Center (Caltech, Goddard)\nfor use of its computational facility. We thank Dr. Theodore\nA. Betley and Larry Henling for crystallographic assistance\nand Dr. David M. Jenkins for assistance with SQUID and\nEPR measurements.", revision_no = "14", abstract = "This paper presents aspects of the coordination chemistry of mono- and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand, [PhBP^(iPr)_3] (where [PhBP^(iPr)_3] = [PhB(CH_2P^(iPr)_2)_3]-). The Mn(II) halide complexes, [PhBP^(iPr)_3]MnCl (1) and [PhBP^(iPr)_3]MnI (2), have been characterized by X-ray diffraction, SQUID magnetometry, and EPR spectroscopy. Compound 2 serves as a precursor to a series of Mn azide, alkyl, and amide species:\u2009 [PhBP^(iPr)_3]Mn(N_3) (3), [PhBP^(iPr)_3]Mn(CH_2Ph) (4), [PhBP^(iPr)_3]Mn(Me) (5), [PhBP^(iPr)_3]Mn(NH(2,6-iPr_2-C_6H_3)) (6), [PhBP^(iPr)_3]Mn(dbabh) (7), and [PhBP^(iPr)_3]Mn(1-Ph(isoindolate)) (8). The complexes 2−8 feature a divalent-metal center and are pseudotetrahedral. They collectively represent an uncommon structural motif for low-coordinate, polyphosphine-supported Mn complexes. Two Mn(I) species have also been prepared. These include the Tl−Mn adduct [PhBP^(iPr)_3]Tl−MnBr(CO)_4 (9) and the octahedral complex [PhBP^(iPr)_3]Mn(CN^tBu)_3 (10). Some of our initial synthetic efforts to generate [PhBP^(iPr)_3]Mn_≡Nx species are briefly described, as are DFT studies that probe the electronic viability of these types of multiply bonded target structures.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47576, title ="Electrocatalytic hydrogen evolution by cobalt complexes of macrocyclic imine ligands", author = "Hu, Xile and Brunschwig, Bruce B.", journal = "Abstracts of Papers of the American Chemical Society", volume = "232", pages = "INOR 205", month = "September", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-080036421", note = "© 2006 American Chemical Society.", revision_no = "16", abstract = "A series of cobalt complexes with macrocyclic imine ligands such as difluoroboryldiglyxomime and [14]-Tetraene-N4 are\nsynthesized for electrochem. hydrogen evolution catalysis. The complexes catalyze the redn. of proton to dihydrogen at\npotentials in the range between -0.55 V to -0.20 V vs. SCE in acidic acetonitrile solns. A correlation between the rates of\nhydrogen evolution and the Co(II/I) potentials of the catalysts is established. The catalysis is triggered by the redn. of\ncobalt(II) precursors to cobalt(I) species, while have been synthesized and studied independently. Attempts are made to\nisolate important catalytic intermediates such as cobalt(III) hydride species.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47578, title ="Electrocatalytic hydrogen evolution with first-row metals: A bimetallic approach", author = "Mendiratta, Arjun and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "232", pages = "INOR 014", month = "September", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-080709798", note = "© 2006 American Chemical Society.", revision_no = "12", abstract = "A new family of dinucleating ligands has been designed and their dicobalt complexes synthesized. The ability of\nthese compds. to catalyze the redn. of protons to dihydrogen has been thoroughly investigated using both\nelectrochem. and spectroscopic methods. Structure-function relationships and mechanistic proposals are\npresented and comparisons are made both to mononuclear analogs and to the bimetallic active site of Fe-only\nhydrogenase.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47581, title ="Probing mechanisms of cobalt-catalyzed dihydrogen production from acidic solutions", author = "Dempsey, Jillian L. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "232", pages = "INOR 252", month = "September", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-081255229", note = "© 2006 American Chemical Society.", revision_no = "13", abstract = "Cobalt macrocycles capable of catalyzing the evolution of dihydrogen from acidic acetonitrile solns. are\nbeing investigated as part of a collaborative solar energy initiative. These metal complexes mediate\nelectrocatalytic dihydrogen prodn. at relatively pos. potentials (up to -0.26 V vs. SCE). The catalysis is\nactivated by the redn. of Co(II) species to a Co(I) intermediate, which likely adds a proton to give a Co(III)\nhydride that reacts further to produce dihydrogen. We are employing laser flash-quench methods coupled\nwith time-resolved spectroscopic monitoring to identify key intermediates in the reaction cycle that leads to\ndihydrogen prodn.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47577, title ="Synthesis of highly luminescent amido-phosphine complexes of Cu, Ag, and Zn", author = "Miller, Alexander J. M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "232", pages = "INOR 843", month = "September", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-080327484", note = "© 2006 American Chemical Society.", revision_no = "11", abstract = "Highly luminescent transition metal complexes have captivated chemists with potential apps. as sensors,\nbiol. imaging agents, and photoreductants. Our recent synth. of an amide-bridged Cu dimer, boasting a long\ndecay lifetime and high quantum yield (11 μs, 58%), has piqued our interest in these areas. Altering\nsupporting ligands--while maintaining the diamond core motif--has marked effects on photoluminescence: while\nthe PNP2Cu2 dimer emits intensely, phosphide-bridged PPP2Cu2 displays very weak luminescence. To help\nelucidate emission mechs., we have prepd. a new PN ligand (PNP sans one supporting phosphine). The PN\nligand presented in this talk provides access to monomeric complexes of type [PN]CuL2, which display\nphotophys. properties. surprisingly similar to their dimeric cousins. These complexes present a unique\nopportunity to tune electronics by subst. of L-type ligands at the metal, or by subst. at the arom. PN backbone.\nThe amidophosphine ligand also yields emissive d10 complexes of Ag, Zn.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47579, title ="Three-coordinate copper platforms that support rapid Cu(II/I) electron transfer and metal-ligand multiple bonding", author = "Mankad, Neal and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "232", pages = "INOR 093", month = "September", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-080912347", note = "© 2006 American Chemical Society.", revision_no = "11", abstract = "Three-coordinate copper ions are important as biol. electron transfer (ET) reagents, and yet synthetic\nthree-coordinate copper systems that mediate reversible ET are rare and the factors that influence their ET\nrates have not been studied. Isolation of three-coordinate copper anilide complexes in the Cu(I) and\nCu(II) oxidn. states supported by bis(phosphino)borate ligands will be described, and measurements of the\nself-exchange ET rates in these systems will be discussed. In addn., the synthesis, spectroscopic\ncharacterization, and reactivity of terminal [bis(phosphino)borate]copper carbene and nitrene species, which\nare implicated as intermediates in copper-catalyzed group transfer reactions, will be presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47586, title ="Unexpected Photoisomerization of a Pincer-type Amido Ligand Leads to Facial Coordination at Pt(IV)", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "45", number = "11", pages = "4316-4318", month = "May", year = "2006", doi = "10.1021/ic052014h ", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-090949354", note = "© 2006 American Chemical Society.\n\nPublished on Web 04/25/2006.\n\nThis work was supported with funds\nprovided by the DOE (PECASE) and the MC2 program in\ncollaboration with BP. S.B.H. thanks the International\nPrecious Metals Institute. Larry Henling provided crystallographic\nassistance.", revision_no = "15", abstract = "The divalent complex (BQA)PtMe undergoes oxidative addition with MeI to afford the octahedral complex cis-(mer-BQA)PtMe_2I {(BQA)- = bis(8-quinolinyl)amide}. When this molecule is irradiated with visible light, it isomerizes to (fac-BQA)PtMe2I, where the BQA ligand adopts an unexpected facial coordination mode. The amide nitrogen in this molecule is sp^3 hybridized and can be easily quarternized with HBF_4, resulting in [H(fac-BQA)PtMe_2I][BF_4], with only minor perturbation to the coordination sphere.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/3508, title ="Complexes of iron and cobalt with new tripodal amido-polyphosphine hybrid ligands", author = "Whited, Matthew T. and Rivard, Eric", journal = "Chemical Communications", volume = "2006", number = "15", pages = "1613-1615", month = "April", year = "2006", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:WHIcc06", note = "© Royal Society of Chemistry 2006 \n\nReceived (in Berkeley, CA, USA) 14th November 2005, Accepted 1st March 2006. First published on the web 10th March 2006 \n\nWe acknowledge the NIH (GM 070757 to JCP) for financial support of this work. MTW is grateful to the Moore Foundation for a graduate fellowship. Neal Mankad and Larry Henling provided crystallographic assistance. \n\nElectronic supplementary information (ESI) available: Synthetic protocols and characterization data; crystallographic data. See DOI: 10.1039/b516046d \n\nDetails of the X-ray diffraction studies: 1, C56.5H61ClFeNP3Si: triclinic, space group P-1, a = 10.8604(17) Å, b = 20.831(3) Å, c = 23.586(4) Å, = 72.318(3)°, = 83.313(3)°, = 89.880(3)°, V = 5046.3(14) Å3, Z = 4, µ(Mo-K) = 0.509 mm–1. 19505 total reflections, 14316 independent (Rint = 8.01%) with I > 2(I); 2a, C51H52ClCoNP3Si: triclinic, space group P-1, a = 12.8058(15) Å, b = 17.612(2) Å, c = 20.505(3) Å, = 93.701(4)°, = 91.736(4)°, = 103.463(4)°, V = 4483.4(10) Å3, Z = 4, µ(Mo-K) = 0.613 mm–1. 17435 total reflections, 9939 independent (Rint = 13.19%) with I > 2(I); 2b, C63H64CoINP3Si: monoclinic, space group P21/c, a = 13.0701(10) Å, b = 19.1387(15) Å, c = 22.1493(16) Å, = 102.943(3)°, V = 5399.8(7) Å3, Z = 4, µ(Mo-K) = 1.042 mm–1. 44992 total reflections, 10823 independent (Rint = 16.66%) with I > 2(I); 3, C45H50ClFeNP2Si: monoclinic, space group P21/n, a = 9.793(3) Å, b = 9.371(3) Å, c = 44.610(12) Å, = 94.581(4)°, V = 4080.9(19) Å3, Z = 4, µ(Mo-K) = 0.575 mm–1. 34419 total reflections, 8553 independent (Rint = 7.69%) with I > 2(I); 4b, C43H54CoINOP2Si: monoclinic, space group P21/c, a = 11.6071(8) Å, b = 21.6762(15) Å, c = 16.8895(12) Å, = 92.988(2)°, V = 4243.6(5) Å3, Z = 4, µ(Mo-K) = 1.268 mm–1. 77696 total reflections, 14707 independent (Rint = 7.91%) with I > 2(I). CCDC 289249–289252. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b516046d", revision_no = "5", abstract = "Divalent complexes of iron and cobalt with new, monoanionic tripodal amido-polyphosphine ligands have been thoroughly characterized, and XRD analysis reveals geometries that are distinct for this class of ligand.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47684, title ="Characterization of the Terminal Iron(IV) Imides {[PhBP^(tBu)_2(pz‘)]Fe^(IV)≡NAd}^+", author = "Thomas, Christine M. and Mankad, Neal P.", journal = "Journal of the American Chemical Society", volume = "128", number = "15", pages = "4956-4957", month = "April", year = "2006", doi = "10.1021/ja0604358", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-164958213", note = "© 2006 American Chemical Society.\n\nReceived January 27, 2006; Publication Date (Web): March 28, 2006.\n\nWe acknowledge Larry Henling for crystallographic\nassistance, and Dr. Mark Mehn for assistance with EPR\nspectroscopy. We thank the NIH for financial support (GM 070757), and N.P.M. is grateful for an NSF graduate fellowship.", revision_no = "12", abstract = "New hybrid bis(phosphine)(pyrazole)borate tripodal ligands ([PhBP^(tBu)_2(pz‘)]-) are reported that support pseudotetrahedral iron in the oxidation states +1, +2, +3, and +4. The higher oxidation states are stabilized by a terminal Fe_≡NR linkage. Of particular interest is the generation and thorough characterization of an S = 1 Fe^(IV)_≡NR^+ imide cation using this new ligand system. The latter species can be observed electrochemically and spectroscopically, and its solid-state crystal structure is reported.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47562, title ="Mid- to high-valent imido and nitrido complexes of iron", author = "Mehn, Mark P. and Peters, Jonas C.", journal = "Journal of Inorganic Biochemistry", volume = "100", number = "4", pages = "634-643", month = "April", year = "2006", doi = "10.1016/j.jinorgbio.2006.01.023 ", issn = "0162-0134", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-142345190", note = "© 2006 Elsevier Inc.\n\nReceived 4 January 2006; received in revised form 11 January 2006; accepted 11 January 2006;\nAvailable online 10 March 2006.\n\nWe thank the NIH for financial support of our research\nthat is related to the theme of this review (GM-070757 to\nJ.C.P. and GM-072291 to M.P.M.).", revision_no = "10", abstract = "This short review summarizes recent advances in the synthesis and reactivity of iron imides and nitrides. Both monometallic and multimetallic assemblies featuring these moieties are discussed. Recent synthetic approaches have led to the availability of new mid- to high-valent iron imides and nitrides, allowing us to begin assembling models to describe the factors influencing their relative stabilities and reactivity patterns.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48279, title ="Coordination chemistry of iron and cobalt supported by hybrid bis(phosphino) pyrazolylborate ligands", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 734", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140811-083353060", note = "© 2006 American Chemical Society.", revision_no = "11", abstract = "Motivated by the unique and versatile late transition metal chem. that has been accessed using the\ntris(phosphino)borate ligands, a new hybrid ligand in which one of the phosphine arms has been replaced\nwith a pyrazolyl moiety has been designed. The coordination chem. of this ligand has been explored using\nboth iron and cobalt. Similar to the tris(phosphine) analogs, the bis(phosphino)pyrazolylborate hybrid\nligands are capable of stabilizing both low valent FeI and CoI complexes as well as FeIII and CoIII terminal\nimides. The synthetic methodol. providing access to this ligand and its derivs. will be discussed and the\ndifferences in coordination chem. imparted by this ligand will be presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47565, title ="Electrocatalytic hydrogen evolution by macrocyclic cobalt and nickel complexes", author = "Hu, Xile and Cossairt, Brandi M.", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 668", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-143617509", note = "© 2006 American Chemical Society.", revision_no = "14", abstract = "A series of cobalt and nickel complexes with macrocyclic imine ligands such as difluoroboryldiglyxomime and [14]-Tetraene-N4 were synthesized for electrochem. hydrogen evolution catalysis. The complexes catalyzed the redn. of proton to dihydrogen at potentials as pos. as -0.26 V vs. SCE in acidic acetonitrile solns. The catalysis by the cobalt complexes was triggered by the redn. of cobalt(II) precursors to cobalt(I) species, while the catalysis by the nickel complexes was mediated by the redn. of nickel(II) precursors to nickel(II) stabilized ligand radicals. The influence of acid strengths on catalysis was discussed: for the cobalt mediated catalysis, using stronger acids as the proton sources gave higher catalytic rates; for the nickel mediated catalysis, the catalytic rates did not vary significantly when using different acids. Structure-function study of the catalysts was also described: more electron-deficient complexes catalyzed hydrogen evolution at more pos. potentials, yet with lower reaction rates.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47566, title ="Geometries that confer unusual spin and oxidation states to first row metals", author = "Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 631", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-143927509", note = "© 2006 American Chemical Society.", revision_no = "11", abstract = "Stereochem. and electronic structure phenomena of the first row transition ions are central issues in\ncoordination chem. These properties are strongly coupled, and the exptl. detn. of one often intimates a great\ndeal about the other. Our group has an ongoing interest in using ligand design and the methods of inorg.\nsynthesis to prep. transition metal complexes that have unusual spin and oxidn. states. For example, we\nhave been able to prep. examples of pseudotetrahedral Fe(II), Fe(III), and Fe(IV) complexes in which the iron\ncenter in each complex is low spin. Likewise, we have reported examples of pseudotetrahedral Co(II) and\nCo(III) complexes that populated low spin ground states. In this talk I will discuss new data for these types\nof systems, and I will also discuss ligands and complexes that have been receiving our lab's attention more\nrecently. For instance, we have been able to prep. a family of dicopper complexes whose copper centers are\npseudotetrahedral that can reversibly access the oxidn. states Cu(I)Cu(I), Cu(1.5)Cu(1.5), and Cu(II)Cu(II)\nwith minimal changes to their overall structural topologies, but large changes to their Cu-Cu distances and their\nCu-X-Cu angles (where X is a bridging ligand) as a function of oxidn. state. By manipulating the auxiliary\nligands of these dicopper systems, their geometric and electronic properties can be perturbed in a fascinating\nway.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47564, title ="New facial amidophosphine ligands: Construction and applications", author = "MacKay, Bruce A. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 732", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-143331272", note = "© 2006 American Chemical Society.", revision_no = "13", abstract = "Amido(bis)phosphines that bind facially to transition metals represent a logical design iteration that offers\ncoordination properties distinct from those of tris(phosphino)borates already studied by our group. Chief\namong these new properties are the electronic effects resulting from a coordinating hard amido donor, esp. via\npossible pi donation. A general synthetic scheme that allows parallel variation of functional groups at the N and\nP donors has been developed, and preliminary coordination chem. is presented and discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48287, title ="Synthesis and reactivity of iron and cobalt complexes supported by new amido-polyphosphine hybrid ligands", author = "Whited, Matthew T. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 812", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140811-090638539", note = "© 2006 American Chemical Society.", revision_no = "10", abstract = "Mixed amide-phosphine hybrid ligands have been shown to support a wide range of transformations on early\ntransition metals, but their coordination chem. with later first-row transition metals has been less explored.\nInspired by the versatility of poly(phosphino)borate ligands, we have developed several new monoanionic\namido-polyphosphine hybrid ligands to support small mol. activation and group transfer reactions on iron\nand cobalt. The ability of these ligands to stabilize low-valent iron and cobalt species will be discussed as\nwell as preliminary investigations into their propensity to support low- to mid-valent group transfer reactivity.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/48280, title ="Two electron redox couples for hydrogen production", author = "Peters, Jonas C. and Hu, Xile", journal = "Abstracts of Papers of the American Chemical Society", volume = "231", pages = "INOR 412", month = "March", year = "2006", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140811-083620961", note = "© 2006 American Chemical Society.", revision_no = "13", abstract = "As part of a collaborative, NSF-supported initiative to help develop the basic chem. needed for producing solar\nfuels, our group has been studying relatively inexpensive, first row transition metals that mediate\nelectrocatalytic hydrogen evolution at unusually pos. potentials. These complexes constitute a severe\nstructural departure from the active sites of hydrogenase enzymes, but are attractive candidates for hydrogen\nevolution because they access reduced states at quite pos. potentials that then trigger the two-electron redn.\nof proton to hydride. The systems are unusual in that they display remarkable tolerance to the typical\nhydrogenase poison, carbon monoxide. Our efforts to more fully map the chem. of these types of systems will\nbe described.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47550, title ="An η^3-H_2SiR_2 Adduct of [{PhB-(CH_2PiPr_2)_3}Fe^(II)H", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "45", number = "5", pages = "776-780", month = "January", year = "2006", doi = "10.1002/anie.200502527", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-085524799", note = "© 2006 Wiley-VCH Verlag GmbH & Co. Article first published online: 21 Dec 2005. Manuscript Revised: 12 Oct 2005.\nManuscript Received: 19 Jul 2005. This work was supported by the NSF (CHE-01232216) and BP (MC2 program). Larry Henling and Dr. Mike Day are acknowledged for crystallographic assistance, and Erin J. Daida is acknowledged for executing several preliminary screen reactions. We also thank the referees of this manuscript for several insightful suggestions.", revision_no = "13", abstract = "Reaction of the high-spin iron(II) alkyl complex [{PhB(CH_2PiPr_2)_3}FeMe] with phenylsilane or mesitylsilane affords unusual diamagnetic silane adducts of the iron(II) hydride [{PhB(CH_2PiPr_2)_3}FeH]. An η^3 bonding mode in these silane adducts has been established by NMR spectroscopy, X-ray crystallography, and DFT methods. Rapid interconversion of the hydride positions in solution via a silylene intermediate is proposed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/24357, title ="High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands", author = "Mehn, Mark P. and Brown, Steven D.", journal = "Dalton Transactions", volume = "10", pages = "1347-1351", month = "January", year = "2006", issn = "1477-9226", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110711-110613327", note = "© 2006 The Royal Society of Chemistry. Received 6th July 2005, Accepted 18th October 2005. First published as an Advance Article on the web 17th November 2005. This work was supported by the National Institutes of Health\n(GM-33162 to L. Q. and GM-070757 to J. C. P.) and the National Science Foundation (CHE-0203346 to C. J. C.) We thank Larry Henling of the Beckman Institute Crystallographic Facility at Caltech for help with refinement of 2. ", revision_no = "18", abstract = "Rare examples of monometallic high-spin and low-spin L_3Fe(H_3BH) complexes have been characterized, where the two L_3 ligands are [Tp^(Ph2)] and [PhBP3] ([Tp^(Ph2)] = [HB(3,5-Ph_2pz)_3]− and [PhBP_3] = [PhB(CH_2PPh_2)_3]−). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers. Despite the differences in spin states, short Fe–B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L_3)Fe(η^4-H_3BH) complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47587, title ="Structural Snapshots of a Flexible Cu_2P_2 Core that Accommodates the Oxidation States Cu^ICu^I, Cu^(1.5)Cu^(1.5), and Cu^(II)Cu^(II)", author = "Mankad, Neal P. and Rivard, Eric", journal = "Journal of the American Chemical Society", volume = "127", number = "46", pages = "16032-16033", month = "November", year = "2005", doi = "10.1021/ja056071l", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-091850419", note = "© 2005 American Chemical Society.\n\nReceived September 2, 2005;Publication Date (Web): October 28, 2005.\n\nWe thank the BP MC^2 program for financial support. N.P.M. was supported by an NSF graduate research fellowship, and E.R. by an NSERC postdoctoral fellowship. Larry Henling, Ted Betley, Brian Leigh, and Dr. Angelo Di Bilio provided\ntechnical assistance. The Zewail group provided access to a near-IR spectrometer. Luminescence measurements were acquired at the Beckman Institute Laser Resource Center.", revision_no = "14", abstract = "The phosphido-bridged dicopper(I) complex {(PPP)Cu}_2 has been synthesized and structurally characterized ([PPP]^- = bis(2-di-iso-propylphosphinophenyl)phosphide). Cyclic voltammetry of {(PPP)Cu}_2 in THF shows fully reversible oxidations at −1.02 V (Cu^(1.5)Cu^(1.5)/Cu^ICu^I) and −0.423 V (Cu^(II)Cu^(II)/Cu^(1.5)Cu^(1.5)). Chemical oxidation of {(PPP)Cu}_2 by one electron yields the class III mixed-valence species [{(PPP)Cu}_2]^+ (EPR, UV−vis). Structural data establish an unexpectedly large change (0.538 Å) in the Cu•••Cu distance upon oxidation state. Oxidation of {(PPP)Cu}_2 by two electrons yields the dication [{(PPP)Cu}_2]^(2+), an antiferromagnetically coupled dicopper(II) complex. Maintenance of a pseudotetrahedral geometry that is midway between a square plane and an ideal tetrahedron at the copper centers, along with a high degree of flexibility at the phosphide hinges, allows for efficient access to Cu^ICu^I, Cu^(1.5)Cu^(1.5), and Cu^(II)Cu^(II) redox states without the need for ligand exchange, substitution, or redistribution processes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47559, title ="Comparative Studies with Zwitterionic Platinum(II) Bis(pyrazolyl)borate and 2,2‘-Bipyridylborate Complexes", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Organometallics", volume = "24", number = "24", pages = "5858-5867", month = "November", year = "2005", doi = "10.1021/om050538j", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-123206635", note = "© 2005 American Chemical Society.\n\nReceived June 28, 2005.\n\nPublication on Web 10/22/2005.\n\nThe authors acknowledge J. Christopher Thomas for insightful discussions. BP (Methane Conversion Cooperative) and the NSF (CHE-01232216) are acknowledged for funding. Larry Henling and Theodore A. Betley are acknowledged for crystallographic assistance.", revision_no = "11", abstract = "A comparison between the mononuclear platinum complexes of three structurally different monoanionic borato ligands is presented:\u2009 [Ph_2B(pyrazolyl)_2]- ([Ph_2B(pz)_2], 1), [4-Ph_3B(2,2‘-bipyridine)]- ([(4-BPh_3)bpy], 2), and [Ph_2B(CH_2PPh_2)_2]- ([Ph_2BP_2], 3). The new bipyridylborate ligand 2 is introduced in this study. The relative trans influence of these ligands has been assessed by comparison of the structural and spectroscopic (NMR) data of the platinum dimethyl complexes [[Ph_2B(pz)_2]Pt(Me)_2][NBu_4] (4), [[(4-BPh_3)bpy]Pt(Me)_2][NBu_4] (5), and [[Ph_2BP_2]Pt(Me)_2][ASN] (6). The neutral complexes [Ph_2B(pz)_2]Pt(Me)(NCCH_3) (7), [Ph_2B(pz)_2]Pt(Me)(CO) (8), [Ph_2B(pz)_2]Pt(Me)(P(C_6F_5)_3) (9), [(4-BPh_3)bpy]Pt(Me)(NCCH_3) (10), [(4-BPh_3)bpy]Pt(Me)(CO) (11), and [(bpy)Pt(Me)(CO)][BPh_4] (12) were prepared, and the carbonyl complexes 8, 11, and 12 provide information pertaining to the relative electron-releasing character of each ligand type. The CO stretching frequencies suggest that the charged borate moiety renders the borato ligands more electron-donating than their neutral analogues. Of the neutral platinum methyl solvento complexes supported by ligands 1, 2, and 3, only those of 1 display very different C−H activation propensities. Upon protonation or methide abstraction in benzene at room temperature, complex 4 rapidly activates two molecules of benzene to generate [[Ph_2B(pz)_2]Pt(Ph)_2][NBu_4] (13). Isotopic scrambling of deuterium into methane in C_6D_6 solvent suggests the intermediacy of a methane σ-adduct in this reaction. The double C−H activation reaction can be halted by addition of acetonitrile to trap the intermediate [Ph_2B(pz)_2]Pt(Ph)(NCCH_3) (14). Complex 3 also displays reactivity toward the benzylic C−H bonds of mesitylene at room temperature to form [Ph_2B(pz)_2]Pt(pzH)(CH_2C_6H_3(CH_3)_2) (15) in modest yield.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47997, title ="Heterolytic H_2 Activation Mediated by Low-Coordinate L_3Fe-(µ-N)-FeL_3 Complexes to Generate Fe(µ-NH)(µ-H)Fe Species", author = "Brown, Steven D. and Mehn, Mark P.", journal = "Journal of the American Chemical Society", volume = "127", number = "38", pages = "13146-13147", month = "September", year = "2005", doi = "10.1021/ja0544509", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-134110901", note = " © 2005 American Chemical Society. Received July 5, 2005. Publication Date (Web): September 3, 2005. We thank the NIH for financial support (GM-070757 to J.C.P., GM-072291 to M.P.M.), and Dr. Angel J. Di Bilio, Lawrence Henling, and Dr. Michael W. Day for assistance. Supporting Information Available: Experimental and characterization data; crystallographic data. This material is available free of\ncharge via the Internet at http://pubs.acs.org.", revision_no = "14", abstract = "The diiron μ-nitride complexes, {L_3Fe^(II)(μ-N)Fe^(II)L_3}- and L_3Fe^(III)(μ-N)Fe^(II)L_3, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L_3 = [PhB(CH_2PPh_2)_3]-). These transformations lead to structurally unique {L_3Fe^(II)(μ-NH)(μ-H)Fe^(II)L^3}- and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3 products. X-ray data establish a marked reduction in the Fe−Fe distance upon H_2 uptake, and spectroscopic data establish both Fe^(II)Fe^(II) species to be diamagnetic, whereas the Fe^(III)Fe^(II) species, L_3Fe^(III)(μ-N)Fe^(II)L_3 and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3, populate doublet ground states with thermally accessible higher spin states.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47549, title ="Coordination chemistry from trigonally coordinated iron platforms: Chemistry relevant to dinitrogen reduction", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "230", pages = "INOR 80", month = "August", year = "2005", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-085417639", note = "© 2005 American Chemical Society.", revision_no = "11", abstract = "Use of the sterically-encumbering tris(phosphino)borate ligand, 1;PhB(CH_2P^iPr_2)_33;-, enabled isolation of tetrahedral iron species\nspanning five oxidn. states, all featuring nitrogenous ligands in the fourth coordination site. Low oxidn. state iron species\n(e.g. Fe^0, Fe^I) readily bind weakly-coordinating, π-acidic ligands (e.g. N_2). The bound dinitrogen can then be further reduced and even functionalized to yield diazenido products (e.g. Fe^(II)(N_2R)). High oxidn. state iron complexes were fashioned through\noxidative group transfer reactions to yield imido/nitrene products (e.g. Fe^(III)(NR)), while decompn. of an iron-ligated, dibenzoazabicycloheptadiene amide yielded a room temp. stable Fe^(IV) nitride complex. The ability to mediate multi-electron\nredox transformations demonstrates the redox flexibility inherent to these iron complexes. We have demonstrated that a\nsingular iron site is capable of stabilizing the span of oxidn. states required for dinitrogen redn. (i.e., Fe^I in Fe(N_2) and Fe^(IV) in Fe≡N) and have isolated and characterized several of the potential intermediates relevant to dinitrogen redn.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47958, title ="Spin-State Tuning at Pseudotetrahedral d^7 Ions: Examining the Structural and Magnetic Phenomena of Four-Coordinate [BP_3]Co^(II)-X Systems", author = "Jenkins, David M. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "127", number = "19", pages = "7148-7165", month = "May", year = "2005", doi = "10.1021/ja045310m", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-084117047", note = "© 2005 American Chemical Society. Received August 3, 2004. Publication Date (Web): April 20, 2005. Financial support from the NSF (CHE-0132216), the DOE (PECASE), and the Alfred P. Sloan Foundation is gratefully acknowledged. D.M.J. was supported by an NSF predoctoral fellowship. The Beckman Institute (Caltech) provided access to a SQUID magnetometer, and the Zewail lab provided access to a NIR spectrophotometer. Dr. Michael Day and Larry Henling provided crystallographic assistance, and Dr. Angel Di Bilio provided EPR assistance. Finally, we are very grateful to Dr. Santiago Alvarez and Jordi Cirera (Universitat de Barcelona, Spain) for help with the\ncontinuous symmetry measurement analysis and a stimulating\nscientific exchange.", revision_no = "13", abstract = "Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate [BP_3] ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhB(CH_2PPh_2)_3]^- and [PhB(CH_2PiPr_2)_3]^-, respectively) are described. Standard ^1H NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes [PhBP_3]CoOSiPh_3 (2), [PhBP_3]CoO(4-^tBu-Ph) (3), [PhBP_3]CoO(C_6F_5) (4), [PhBP_3]CoSPh (5), [PhBP_3]CoS(2,6-Me_2-Ph) (6), [PhBP_3]CoS(2,4,6-iPr_3-Ph) (7), [PhBP_3]CoS(2,4,6-^tBu_3-Ph) (8), [PhBP_3]CoSSiPh_3 (9), [PhBP_3]CoOSi(4-NMe_2-Ph)_3 (10), [PhBP_3]CoOSi(4-CF_3-Ph)_3 (11), [PhBP_3]CoOCPh_3 (12), [PhBPiPr_3]CoOSiPh_3 (14), and [PhBPiPr_3]CoSSiPh_3 (15). The low-temperature solid-state crystal structures of 2, 3, 5−10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data demonstrate that both S = 1/2 and S = 3/2 ground-state electronic configurations are accessible for the umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (η^1 versus η^3), and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S = 1/2 ground-state exclusively. For those four-coordinate complexes that populate S = 1/2 ground states, X-ray data show two Co−P bond distances that are invariably shorter than a third Co−P bond. The pseudotetrahedral siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The diamagnetic cobalt(III) complex {[PhBP_3]CoOSiPh_3}{BAr_4} ({16}{BAr_4}) (Ar = Ph or 3,5-(CF_3)_2−C_6H_3) has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT) for complexes 2, 6, and {16}^+ support the notion of a close electronic structure relationship between these four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/24954, title ="Synthesis and characterization of cationic iron complexes supported by the neutral ligands NP^(i-Pr)_3, NArP^(i-Pr)_3, and NS^(t-Bu)_3^1", author = "MacBeth, Cora E. and Harkins, Seth B.", journal = "Canadian Journal of Chemistry", volume = "83", number = "4", pages = "332-340", month = "April", year = "2005", issn = "0008-4042", url = "https://resolver.caltech.edu/CaltechAUTHORS:20110819-090808091", note = "© 2005 NRC Canada. Received 9 September 2004. \nPublished on the NRC Research Press Web site at http://canjchem.nrc.ca on 23 March 2005.\nWe acknowledge the DOE (Department of Energy) (PECASE), the NSF (National Science Foundation) (CHE-0132216), and the Beckman Institute Senior Research Fellows Program (CEM) for financial support. Larry Henling and Dr. Michael Day provided crystallographic assistance. ", revision_no = "16", abstract = "This paper compares the local geometries, spin states, and redox properties of a series of iron complexes supported by neutral, tetradentate NP_3 (tris(phosphine)amine) and NS_3 (tris(thioether)amine) ligands. Our consideration of an Fe-mediated N_2 fixation scheme similar to that proposed by Chatt for molybdenum motivates our interest in systems\nof these types. This report specifically describes the synthesis and characterization of cationic Fe(II) chloride\ncomplexes supported by the neutral ligands NP^(i-Pr)_3 (NP^(i-Pr)_3 = [N(CH_2CH_2P-i-Pr_2_)3]), NArP^(i-Pr)_3 (NArP^(i-Pr)_3 = [N(2-diisopropylphosphine-4-methylphenyl)_3]), and NS^(t-Bu)_3 (NS^(t-Bu)_3 = [N(CH_2CH_2S-t-Bu)_3]). The solid-state structures, electrochemistry,and magnetic properties of these complexes are reported. Whereas the NPV(i-Pr)_3 and NArP^(i-Pr)_3 ligands provide pseudotetrahedral S = 2 ferrous cations [Fe(NP^(i-Pr)_3)Cl]PF_6 (1[PF_6]) and [Fe(NArP^(i-Pr)_3)Cl]BPh_4 (2[BPh_4]) featuring a long Fe—N bond distance, the NS^(t-Bu)_3 ligand gives rise to a trigonal bipyramidal structure with a S = 1 ground state and a much shorter Fe–N interaction. The complexes 1[BPh_4] and 2[BPh_4] can be reduced under CO to give rise to the\nfive-coordinate Fe(I) monocarbonyls [Fe(NP^(i-Pr)_3)CO]BPh_4 (4[BPh_4]) and [Fe(NAr^(Pi-Pr)_3)CO]BPh_4 (5[BPh_4]). The solidstate structures and electrochemistry of 4[BPh_4] and 5[BPh_4] are described, as is the EPR spectrum of 4[BPh_4]. The synthesis and characterization of the hydride–dinitrogen complex [Fe(NP^(i-Pr)_3)(N_2)(H)]PF_6 (6[PF_6]) has also been accomplished and its properties are also reported.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47551, title ="Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands", author = "Li, Jian and Djurovich, Peter I.", journal = "Inorganic Chemistry", volume = "44", number = "6", pages = "1713-1727", month = "March", year = "2005", doi = "10.1021/ic048599h", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-091550683", note = "© 2005 American Chemical Society. \n\nReceived October 7, 2004. Publication Date (Web): February 12, 2005. \n\nThe authors thank the Universal Display Corporation, the Defense Advanced Research Projects Agency, and the National Science Foundation for their financial support.", revision_no = "18", abstract = "The synthesis and photophysical characterization of a series of (N,C^2‘-(2-para-tolylpyridyl))_2Ir(LL‘) [(tpy)_2Ir(LL‘)] (LL‘ = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)_2Ir(LL‘)] (dfppy = N,C^2‘-2-(4‘,6‘-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)_2Ir(CN)_2- and (ppy)_2Ir(NCS)_2- (ppy = N,C^2‘-2-phenylpyridyl). The (tpy)_2Ir(PPh_2CH_2)_2BPh_2 and [(tpy)_2Ir(CN-t-Bu)_2](CF_3SO_3) complexes have been structurally characterized by X-ray crystallography. The Ir−C_(aryl) bond lengths in (tpy)_2Ir(CN-t-Bu)_2^+ (2.047(5) and 2.072(5) Å) and (tpy)_2Ir(PPh_2CH_2)_2BPh_2 (2.047(9) and 2.057(9) Å) are longer than their counterparts in (tpy)_2Ir(acac) (1.982(6) and 1.985(7) Å). Density functional theory calculations carried out on (ppy)_2Ir(CN-Me)_2^+ show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-π and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-π orbitals. Electrochemical analysis of the (tpy)_2Ir(LL‘) complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL‘ ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T_1) is a triplet ligand-centered state (^3LC) on the cyclometalating ligand admixed with ^1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the ^1MLCT state energy mainly by changing the HOMO energy. Destabilization of the ^1MLCT state results in less ^1MLCT character mixed into the T_1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k_(nr)) (k_(nr) = nonradiative decay rate). Decreased ^1MLCT character in the T_1 state also increases the Huang−Rhys factors in the emission spectra, decreases the extinction coefficient of the T_1 transition, and consequently decreases the radiative decay rates (k_r). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k_r) or extinction coefficient (ε) on (1/ΔE)^2 has been demonstrated, where ΔE is the energy difference between the ^1MLCT and ^3LC transitions. A value of 200 cm^(-1) for the spin−orbital coupling matrix element ‹^3LC|H_(SO)|^1MLCT› of the (tpy)_2Ir(LL‘) complexes can be deduced from this linear relationship. The (fppy)_2Ir(LL‘) complexes with corresponding ancillary ligands display similar trends in excited-state properties.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47560, title ="Exploratory studies of iron and cobalt complexes supported by strong-field tripodal (phosphino)borate ligands", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "229", pages = "INOR 794", month = "March", year = "2005", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-130315618", note = "© 2005 American Chemical Society.", revision_no = "11", abstract = "Recent work within our lab has focused on the coordination chem. of late transition metal complexes supported by the\nstrong-field, tripodal phosphino(borate) ligands [PhB(CH_2PPh_2)_3]- and [PhB(CH_2PiPr_2)_3]-. These platforms have been shown to\nstabilize a variety of metal oxidn. states and, moreover, a no. of unique coordination compds. featuring metal-nitrogen multiple\nbonds. With this in mind, the reactivity of low valent iron and cobalt complexes with substrates such as diazoalkanes and\nelemental phosphorous (P_4) will be presented and avenues to metal-phosphorous and/or metal-carbon multiple bonds will be\ndiscussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47568, title ="Highly emissive, redox active amido-bridged Cu_2N_2 diamond core complexes supported by phosphine ligands", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "229", pages = "INOR 403", month = "March", year = "2005", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-145420932", note = "© 2005 American Chemical Society.", revision_no = "11", abstract = "A series of novel Cu_2N_2 diamond core structures supported by P_2N- ligands have been prepd. These\ndiamond core systems exhibit reversible 1-electron redox processes between the Cu^1Cu^1, [CuCu]^+, [CuCu]^2+\nstates. The solid-state mol. structures of each show that minimal overall structural reorganization occurs,\nthough minor core distortions are present. Addnl., the Cu^1Cu^1, form is highly emissive (φ > 65%, τ > 10 μs)\nand gives rise to interesting photoreductive chem. The magnetic, electrochem. and spectroscopic data, as well\nas initial reactivity studies, will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47561, title ="Synthesis and properties of new dicopper diamond core structures", author = "Neal, Mankad and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "229", pages = "INOR 797", month = "March", year = "2005", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-130732636", note = "© 2005 American Chemical Society.", revision_no = "10", abstract = "Recently, our group has synthesized amido-bridged Cu_2N_2 dicopper complexes supported by anionic [SNS]-\nand [PNP]- tridentate ligands. These species exhibit interesting electrochem. and photophys. properties. For\nexample, we have shown that the dinuclear complexes {[SNS]Cu^I}_2 and {[SNS]Cu^(1.5)}_2+ are related by a fully\nreversible one-electron redox process characterized by minimal structural reorganization between the two redox\nforms. More recently, we have prepd. highly emissive {[PNP]Cu^I}_2 derivs. that are also redox active. In this\ntalk I will discuss recent efforts to extend these Cu_2N_2 diamond core structures to related Cu_2P_2 structures\nin which the Cu to Cu distance is significantly increased. Complexes of these types will allow us to probe\nstructure/function relationships in greater detail, which is of particular interest to our understanding of the\nfascinating electrochem. and photophys. properties of the Cu\n_2N_2 systems.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47567, title ="Synthesis of novel cobalt imido and bridging nitride complexes", author = "Jenkins, David M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "229", pages = "INOR 801", month = "March", year = "2005", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-145100810", note = "© 2005 American Chemical Society.", revision_no = "11", abstract = "Late metal metal-ligand multiple bonds are of interest due to their novel structural motifs and their ability to participate in group\ntransfer reactions. Recent work has included the synthesis of new late transition metal imido complexes (M=NR or M≡NR)\ninvolving iron, cobalt and nickel. As part of our contribution to this area of research, we have prepd. a no. of cobalt imido complexes which feature the tripodal auxiliary ligand [PhBP_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-); these imides were obtained via\nthe oxidative degrdn. of org. azides. Addnl., a μ_2- bridging nitride complex of cobalt was prepd. by two routes and the\nmagnetic properties and bonding of this bimetallic complex will be discussed. Furthermore, the potential of these species to\nundergo group transfer chem. will be addressed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47571, title ="A Highly Emissive Cu_2N_2 Diamond Core Complex Supported by a [PNP]^- Ligand", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "127", number = "7", pages = "2030-2031", month = "February", year = "2005", doi = "10.1021/ja043092r", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-160417130", note = "© 2005 American Chemical Society.\n\nReceived November 16, 2004.\n\nThis work was supported with funds provided by the NSF (CHE-01232216)) and the MC2 program in collaboration with BP. We acknowledge Dr. Jennifer C. Lee for technical assistance with the lifetime measurements, and Larry Henling for crystallographic assistance. Dr. Jay R. Winkler provided\nnumerous insightful discussions", revision_no = "13", abstract = "A Cu2N2 diamond core structure, {(PNP)CuI}_(2) (2), supported by a [PNP]- ligand (1) ([PNP]^(-) = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (τ > 10 μs) and an unusually high quantum yield (φ > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. −3.2 V vs Fc^(+)/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47995, title ="Ground-State Singlet L_3Fe-(í-N)-FeL_3 and L_3Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers", author = "Brown, Steven D. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "127", number = "6", pages = "1913-1923", month = "February", year = "2005", doi = "10.1021/ja0453073", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-132349667", note = "© 2005 American Chemical Society. \n\nReceived August 4, 2004. Publication Date (Web): January 19, 2005. \n\nFinancial support from the NSF (CHE-0132216), the DOE (PECASE), and the Alfred P. Sloan Foundation is gratefully acknowledged. We thank the Beckman Institute (Caltech) for use of the SQUID magnetometer, and Larry M. Henling and Theodore A. Betley for assistance with crystallographic studies. The Zewail group provided access to a Cary 500 UV/vis/NIR spectrophotometer.", revision_no = "21", abstract = "Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP_]Fe(μ-1,3-N_3)}_2 (2) ([PhBP_3] = [PhB(CH_2PPh_2)_3]-), yields the diamagnetic bridging nitride species [{[PhBP_3]Fe}_2(μ-N)][Na(THF)_5] (3). The Fe−N−Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135°, and the short Fe−N bond distances (Fe−N_(av) ≈ 1.70 Å) suggest substantial Fe−N multiple bond character. The diamagnetic imide complex {[PhBP_3]Fe^(II)_≡N(1-Ad)}{^nBu_4N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP_3]Fe^(III)_≡N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/4825, title ="Electrocatalytic hydrogen evolution by cobalt difluoroboryl-diglyoximate complexes", author = "Hu, Xile and Cossairt, Brandi M.", journal = "Chemical Communications", volume = "2005", number = "37", pages = "4723-4725", month = "January", year = "2005", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:HUXcc05", note = "© Royal Society of Chemistry 2005 \n\nReceived (in Berkeley, CA, USA) 28th June 2005, Accepted 29th July 2005. First published on the web 23rd August 2005 \n\nFor financial support we acknowledge the NSF through a Chemical Bonding Center (CBC). We also thank Prof. Alex Sessions for his generous help with the gas chromatography experiments.", revision_no = "17", abstract = "In the presence of moderately strong acids in CH3CN, cobalt complexes with BF2-bridged diglyoxime ligands are active catalysts for the reduction of protons to H2 at potentials as positive as –0.28 V vs. SCE.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47583, title ="Synthetic, Structural, and Mechanistic Aspects of an Amine Activation Process Mediated at a Zwitterionic Pd(II) Center", author = "Lu, Connie C. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "126", number = "48", pages = "15818-15832", month = "December", year = "2004", doi = "10.1021/ja046415s", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-084211159", note = "© 2004 American Chemical Society.\n\nReceived June 17, 2004.\n\nPublication Date (Web): November 10, 2004.\n\nThis work was supported by the NSF (CHE-0132216), the ACS Petroleum Research Fund, and the Dreyfus Foundation. The authors thank Theodore Betley, Larry Henling, and Dr. Michael Day for crystallographic assistance. C.C.L. thanks the Department of Defense for a Graduate Research Fellowship.", revision_no = "16", abstract = "A zwitterionic palladium complex [[Ph_(2)BP_(2)]Pd(THF)_(2)][OTf] (1) (where [Ph_(2)BP_(2)] = [Ph_(2_B(CH_(2)PPh_(2))_(2)]-) reacts with trialkylamines to activate a C−H bond adjacent to the amine N atom, thereby producing iminium adduct complexes [Ph_(2)BP_(2)]Pd(N,C:η^(2)-NR_(2)CHR‘). In all cases examined the amine activation process is selective for the secondary C−H bond position adjacent to the N atom. These palladacycles undergo facile β-hydride elimination/olefin reinsertion processes as evident from deuterium scrambling studies and chemical trap studies. The kinetics of the amine activation process was explored, and β-hydride elimination appears to be the rate-limiting step. A large kinetic deuterium isotope effect for the amine activation process is evident. The reaction profile in less polar solvents such as benzene and toluene is different at room temperature and leads to dimeric {[Ph_(2)BP_(2)]Pd}_(2) (4) as the dominant palladium product. Low-temperature toluene-d8 experiments proceed more cleanly, and intermediates assigned as [Ph_(2)BP_(2)]Pd(NEt_(3))(OTf) and the iminium hydride species [[Ph_(2)BP_(2)]Pd(H)(Et_(2)NCHCH_(3))][OTf] are directly observed. The complex (Ph2SiP2)Pd(OTf)2 (14) was also studied for amine activation and generates dimeric [(Ph_(2)SiP_(2))Pd]_(2)[OTf]_(2) (16) as the dominant palladium product. These collective data are discussed with respect to the mechanism of the amine activation and, in particular, the influence that solvent polarity and charge have on the overall reaction profile. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47585, title ="Structural and spectroscopic studies of three-coordinate copper(I) supported by bis(phosphino)borate ligands", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Polyhedron", volume = "23", number = "17", pages = "2901-2913", month = "November", year = "2004", doi = "10.1016/j.poly.2004.08.008", issn = "0277-5387", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-084937391", note = "© 2004 Elsevier Ltd.\n\nReceived 1 July 2004; accepted 25 August 2004; Available online 28 October 2004.\n\nFinancial support from the NSF (CHE-0132216) and the DOE (PECASE) is acknowledged. Larry Henling provided assistance with X-ray crystallography. Christine Thomas generously provided a sample of PhB(CH2PtBu2)2. J.C.T. is grateful to the NSF and to the Moore Foundation for graduate research\nfellowships.\nDedicated to Professor Malcolm H. Green, an inspiring organometallic chemist.", revision_no = "9", abstract = "The bis(phosphino)borates [(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)][ASN] (1[ASN]; ASN = 5-azonia-spiro[4.4]nonane), [Ph_(2)B(CH_(2)P^(i)Pr_(2))_(2)]Li(THF)2 (2[Li]), and [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]^− (3[Li] and 3[Tl]) react with copper(I) reagents to generate anionic and neutral complexes. Reaction of 1[ASN] with CuI provides the 1:1 adduct [[(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)]CuI][ASN] (4). CuCl reacts with 3[Li] in THF to afford [[(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]CuCl][Li(THF)_(4)] (5), along with a minor byproduct, the three-coordinate bis(phosphine) adduct {(3,5-Me_(2)Ph)B(CH_(2)P^(t)Bu_(2))_(2)}CuCl (6). An analogous neutral complex, {PhB(CH_(2)P^(t)Bu_(2))_(2)}CuCl (7), can be synthesized directly by reacting CuCl with the neutral phosphino borane PhB(CH_(2)P^(t)Bu_(2))_(2). Reaction of the substituted copper(I) halide Me2S · CuBr with 3[Tl] provides the anionic dimer complex [{[(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]Cu}_(2)(μ-Br)]Tl (8). The neutral solvent adducts [Ph_(2)B(CH_(2)P^(i)Pr_(2))_(2)]Cu(NCCH_(3)) (9) and [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu)_(2))_(2)]Cu(NCCH_(3)) (10) are prepared by the reactions between [Cu(CH_(3)CN)_(4)][PF_(6)] and 2[Li] or 3[Li], respectively. Neutral 10 is a useful precursor to several other neutral species via substitution of the labile acetonitrile ligand, including [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]Cu(PMe_(2)Ph)(11), [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]Cu(Sdouble bond; length as m-dashPMe3) (12), [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)] Cu(2,6-lutidine) (13), and [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]Cu(CN^(t)Bu) (14). Attempts to isolate a neutral monocarbonyl adduct, [(3,5-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)]Cu(CO) (15), are frustrated by the high lability of the CO ligand. The solid-state crystal structures of complexes 6, 8, 9, 10, and 11 have been determined and are described.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47685, title ="Considering Fe^(II/IV) Redox Processes as Mechanistically Relevant to the Catalytic Hydrogenation of Olefins by [PhBP^(iPr)_3]Fe-H_x Species", author = "Daida, Erin J. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "43", number = "23", pages = "7474-7485", month = "October", year = "2004", doi = "10.1021/ic0488583", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140731-074557924", note = "© 2004 American Chemical Society. Publication Date (Web): October 14, 2004. Received August 18, 2004. This work was supported by the NSF (CHE-01232216) and the DOE (PECASE). E.J.D. is grateful for the Laszlo Zechmeister Fellowship (Caltech). Lawrence Henling and Theodore A. Betley are acknowledged for crystallographic assistance. We thank Paul J. Chirik for sharing results prior to publication.", revision_no = "13", abstract = "Several coordinatively unsaturated pseudotetrahedral iron(II) precursors, [PhBP^(iPr)_3]Fe−R ([PhBP^(iPr)_3] = [PhB(CH2P^(iPr)_2)_3]^-; R = Me (2), R = CH_2Ph (3), R = CH_2CMe_3 (4)) have been prepared from [PhBP^(iPr)_3]FeCl (1) that serve as precatalysts for the room-temperature hydrogenation of unsaturated hydrocarbons (e.g., ethylene, styrene, 2-pentyne) under atmospheric H_2 pressure. The solid-state crystal structures of 2 and 3 are presented. To gain mechanistic insight into the nature of these hydrogenation reactions, a number of [PhBP^(iPr)_3]-supported iron hydrides were prepared and studied. Room-temperature hydrogenation of alkyls 2−4 in the presence of a trapping phosphine ligand affords the iron(IV) trihydride species [PhBP^(iPr)_3]Fe(H)_3(PR_3) (PR_3 = PMe_3 (5); PR_3 = PEt_3 (6); PR3 = PMePh_2 (7)). These spectroscopically well-defined trihydrides undergo hydrogen loss to varying degrees in solution, and for the case of 7, this process leads to the structurally identified Fe(II) hydride product [PhBP^(iPr)_3]Fe(H)(PMePh_2) (9). Attempts to prepare 9 by addition of LiEt_3BH to 1 instead lead to the Fe(I) reduction product [PhBP^(iPr)_3]Fe(PMePh_2) (10). The independent preparations of the Fe(II) monohydride complex [PhBP^(iPr)_3]FeII(H)(PMe_3) (11) and the Fe(I) phosphine adduct [PhBP^(iPr)_3]Fe(PMe_3) (8) are described. The solid-state crystal structures of trihydride 5, monohydride 11, and 8 are compared and demonstrate relatively little structural reorganization with respect to the P_3Fe−P‘ core motif as a function of the iron center's formal oxidation state. Although paramagnetic 11 (S = 1) is quantitatively converted to the diamagnetic trihydride 5 under H_2, the Fe(I) complex 8 (S = 3/2) is inert toward atmospheric H_2. Complex 10 is likewise inert toward H_2. Trihydrides 5 and 6 also serve as hydrogenation precatalysts, albeit at slower rates than that for the benzyl complex 3 because of a rate-contributing phosphine dependence. That these hydrogenations appear to proceed via well-defined olefin insertion steps into an Fe−H linkage is indicated by the reaction between trihydride 5 and ethylene, which cleanly produces the ethyl complex [PhBP^(iPr)_3]Fe(CH_2CH_3) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47584, title ="Vibrational Spectroscopy and Analysis of Pseudo-tetrahedral Complexes with Metal Imido Bonds", author = "Mehn, Mark P. and Brown, Steven D.", journal = "Inorganic Chemistry", volume = "45", number = "18", pages = "7417-7427", month = "September", year = "2004", doi = "10.1021/ic060670r", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-084256241", note = "© 2006 American Chemical Society.\n\nReceived April 19, 2006.\n\nThis work was supported by the National Institutes of Health at the University of Minnesota (GM-33162 to L.Q. and Predoctoral Traineeship GM-08700 to M.P.M.), and at the California Institute of Technology (GM-070757 to J.C.P. and Postdoctoral Fellowship GM-072291 to M.P.M.). D.M.J. was supported by a National Science Foundation Predoctoral Fellowship. The authors are grateful to Dr. Michael W. Day and Larry M. Henling for technical assistance with the crystallographic portion of this work.", revision_no = "15", abstract = "A number of assignments have been previously posited for the metal−nitrogen stretch (ν(M-NR)), the N−R stretch (ν(MN−R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP_(3)] ([PhBP_(3)] = [PhB(CH_(2)PPh_(2))_(3)]^(-)) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP_(3)]Fe≡N^(t)Bu and [PhBP_(3)]Co≡N^(t)Bu. Pseudo-tetrahedral metal imides of these types exhibit short bond lengths (ca. 1.65 Å) and nearly linear angles about the M−N−C linkages, indicative of multiple bond character. Furthermore, these compounds give rise to intense, low-energy visible absorptions. Both the position and the intensity of the optical bands in the [PhBP_(3)]M≡NR complexes depend on whether the substituent is an alkyl or aryl group. Excitation into the low-energy bands of [PhBP_(3)]Fe≡N^(t)Bu gives rise to two Raman features at 1104 and 1233 cm^(-1), both of which are sensitive to ^(15)N and ^(2)H labeling. The isotope labeling suggests the 1104 cm^(-1) mode has the greatest Fe−N stretching character, while the 1233 cm^(-1) mode is affected to a lesser extent by ^(15)N substitution. The spectra of the deuterium-labeled imides further support this assertion. The data demonstrate that the observed peaks are not simple diatomic stretching modes but are extensively coupled to the vibrations of the ancillary organic group. Therefore, describing these complexes as simple diatomic or even triatomic oscillators is an oversimplification. Analogous studies of the corresponding cobalt(III) complex lead to a similar set of isotopically sensitive resonances at 1103 and 1238 cm^(-1), corroborating the assignments made in the iron imides. Very minimal changes in the vibrational frequencies are observed upon replacement of cobalt(III) for iron(III), suggesting similar force constants for the two compounds. This is consistent with the previously proposed electronic structure model in which the added electron resides in a relatively nonbonding orbital. Replacement of the tBu group with a phenyl ring leads to a significantly more complicated resonance Raman spectrum, presumably due to coupling with the vibrations of the phenyl ring. Polarization studies demonstrate that the observed modes have A1 symmetry. In this case, a clearer resonance enhancement of the signals is observed, supporting a charge transfer designation for the electronic transitions. A series of isotope-labeling experiments has been carried out, and the modes with the greatest metal−nitrogen stretching character have been assigned to peaks at 960 and 1300 cm^(-1) in both the iron and cobalt [PhBP_(3)]M≡NPh complexes. These results are consistent with a multiple M−N bond for these metal imides.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47932, title ="The Coordination Chemistry of “[BP_3]NiX” Platforms: Targeting Low-Valent Nickel Sources as Promising Candidates to L_3Ni=E and L_3Ni≡E Linkages", author = "MacBeth, Cora E. and Thomas, J. Christopher", journal = "Inorganic Chemistry", volume = "43", number = "15", pages = "4645-4662", month = "July", year = "2004", doi = "10.1021/ic049936p", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140804-151216151", note = "© 2004 American Chemical Society. Received January 15, 2004. Publication Date (Web): June 25, 2004. We acknowledge the DOE (PECASE) and the NSF (CHE-0132216) for financial support of this work. Larry Henling, Smith Nielsen, and Eric Peters provided technical assistance. We are grateful to Professors Peter T. Wolczanski and Daniel Rabinovich for providing insightful discussions. We also thank Professor Clifford Kubiak for disclosing results prior to publication. The Beckman Institute Senior Research Fellows Program (CEM), the Moore Foundation (J.C.T.), the NSF (J.C.T.), and the DOD (T.A.B.) are each acknowledged for fellowship support.", revision_no = "12", abstract = "A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-, [PhBP^(iPr)_3] = [PhB(CH_2PiPr_2)_3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP_3]NiCl (1), [PhBP_3]NiI (2), and [PhBP^(iPr)_3]NiCl (3) were prepared by the metalation of [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2NiCl_2, NiI_2, and (DME)NiCl_2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP_3]Ni(N_3) (4), [PhBP_3]Ni(OSiPh_3) (5), [PhBP_3]Ni(O-p-tBu-Ph) (6), and [PhBP_3]Ni(S-p-tBu-Ph) (7). Complexes 2−5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, {[PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]NiCl} (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP_3]Ni(η^2-CH_2PPh_2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [κ^2-PhBP^(iPr)_3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP_3]Ni(PPh_3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe_3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBP^(iPr)_3]Ni(PMe_3) (14) and [PhBP^(iPr)_3]Ni(CN^tBu) (15), respectively. The reactivity of these [PhBP_3]Ni^IL and [PhBP^(iPr)_3]NiI^L complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP_3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[κ^2-PhBP_3]Ni(CO)_2][^nBu_4N] (17) and [[κ^2-PhBP^(iPr)_3]Ni(CO)_2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2Ni(CO)_2 in the presence of R_4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as {[PhBP_3]Ni(CO)}{^nBu_4N}, although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(NMe), and [PhBP^(iPr)_3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47971, title ="A Tetrahedrally Coordinated L_3Fe−N_x Platform that Accommodates Terminal Nitride (Fe^(IV)_≡N) and Dinitrogen (Fe^I−N_2−Fe^I) Ligands", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "126", number = "20", pages = "6252-6254", month = "May", year = "2004", doi = "10.1021/ja048713v", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-111309938", note = "© 2004 American Chemical Society. Received March 5, 2004. Publication Date (Web): April 30, 2004. This work was supported by the NSF (CHE-01232216), the Sloan Foundation, and the Dreyfus foundation. T.A.B. is grateful for a graduate research fellowship from the Department of Defense. Professor Seth N. Brown provided several stimulating discussions on the topic of d_z2.", revision_no = "13", abstract = "A tetrahedrally coordinated L_3Fe−N_x platform that accommodates both terminal nitride (L^3Fe^(IV)_≡N) and dinitrogen (L_3Fe^I−N_2−Fe^IL_3) functionalities is described. The diamagnetic L_3Fe^(IV)_≡N species featured has been characterized in solution under ambient conditions by multinuclear NMR (^1H, ^(31)P, and ^(15)N) and infrared spectroscopy. The electronic structure of the title complex has also been explored using DFT. The terminal nitride complex oxidatively couples to generate the previously reported L_3Fe^I−N_2−Fe^IL_3 species. This reaction constitutes a six-electron transformation mediated by two iron centers. Reductive protonation of the nitride complex releases NH_3 as a significant reaction product.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47589, title ="Hydrogenolysis of [PhBP_3]Fe≡N-p-tolyl:\u2009 Probing the Reactivity of an Iron Imide with H_2", author = "Brown, Steven D. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "126", number = "14", pages = "4538-4539", month = "April", year = "2004", doi = "10.1021/ja0399122 ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-094918980", note = "© 2004 American Chemical Society.\n\nReceived December 3, 2003.\nPublication Date (Web): March 23, 2004.\n\nWe thank the NSF (CHE-01232216) and\nthe DOE (PECASE) for financial support. The authors acknowledge\nDavid M. Jenkins and Larry M. Henling for technical assistance.", revision_no = "18", abstract = "This paper describes the reductive hydrogenolysis of a low-spin (S = 1/2) iron(III) imide. Pseudotetrahedral [PhBP_3]Fe^(III)≡N-p-tolyl is reduced by hydrogen at ambient temperature and pressure in benzene solution. The reduction appears to proceed in a stepwise fashion. An intermediate S = 2 iron(II) anilide, [PhBP_3]Fe(N(H)-p-tolyl), is observed and has been independently generated and structurally characterized. Prolonged hydrogenolysis in benzene results in the complete hydrogenolysis of the Fe−N linkage to release H2N-p-tolyl. The major iron-containing product formed from this step is the diamagnetic cyclohexadienyl complex, [PhBP_3]Fe(η^5-cyclohexadienyl), which has also been independently prepared and structurally characterized. Evidence is presented to suggest that the final [PhBP_3]Fe(η^5-cyclohexadienyl) product is formed via benzene insertion into a reactive [PhBP_3]Fe^(II)-H intermediate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47543, title ="Chemistry of tris(phosphino)borate supported iron compounds featuring metal-nitrogen multiple bonding", author = "Brown, Steven D. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 511", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-074831520", note = "© 2004 American Chemical Society.", revision_no = "12", abstract = "Iron compounds featuring metal-ligand multiple bonds have been postulated in\nboth biological and catalytic processes. Synthetically, however, isolated\nexamples of such compounds have remained relatively elusive. To this extent,\nwe have employed the anionic tris(phosphino)borate ligand [PhB(CH_2PPh_2)_3]- for\nthe synthesis of iron compounds featuring metal-nitrogen multiple bonds;\nincluding monomeric terminal imides. The synthesis, properties, and reactivity of\nthese compounds will be presented, highlighting both group transfer chemistry\nand the hydrogenolytic cleavage of an iron-nitrogen triple bond.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47529, title ="Dinitrogen chemistry from trigonally coordinated iron and cobalt platforms", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 506", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-135836434", note = "© 2004 American Chemical Society.", revision_no = "12", abstract = "The chemistry of low-valent iron and cobalt has been slow to develop, due\nlargely in part to a lack of ligand scaffolds that can stabilize these species.\nUsing the anionic tris(phosphino)borate ligand, [PhB(CH_2PiPr_2)_3]-, we have\nstabilized reactive complexes of the type [P_3M-L] where L can be varied from\nweakly-donating π-acidic ligands (e.g. N_2), to strongly π-basic ligands where\nmultiple bonding occurs between the metal and L (e.g. NR ^2-). In addition to\nstabilizing a broad range of ligand types in a variety of oxidation states, the\n[P_3M] template has been shown to mediate the coordination and activation of\ndinitrogen on both iron and cobalt. The reaction chemistry of the [P_3M-L]\nscaffolds will be presented in addition to synthetic strategies targeting high-valent\nspecies (M^(IV), L=N ^3-).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47536, title ="Four-coordinate, 14-electron iron alkyl complexes supported by the tripodal phosphine ligand [PhB(CH_2PiPr_2)_3]^-", author = "Daida, Erin J. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 285", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-154320758", note = "© 2004 American Chemical Society.", revision_no = "12", abstract = "Recent work in our laboratories has focused on the use of tridentate, anionic\nphosphine ligands to promote novel reactivity of the late transition metals. The\nhighly electron-releasing, sterically demanding tris(diisopropylphosphino)borate\nligand, [PhBP^(iPr3)] ([PhBP^(iPr3)]=[PhB(CH_2PiPr_2)_3]^- has been shown to stabilize\ncoordinatively unsaturated iron complexes with a variety of oxidation states.\nWhile 18-electron iron alkyl species are known, four-coordinate, low-electron-count\niron alkyl complexes are relatively rare. It was of interest to synthesize\nmonoalkyl complexes on the [PhBP^(ipr3)]Fe platform because of their potential\nuse as carbene precursors. A series of four-coordinate, 14-electron iron(ll) alkyl\ncomplexes have been synthesized. The complexes have been characterized by a\nvariety of spectroscopic and electrochemical methods as well as x-ray crystallography.\nThe characterization and reactivity of the alkyl complexes will be\ndescribed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47535, title ="Redox active amido-bridged Cu_2N_2 diamond core complexes supported by thioether and phosphine ligands", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 519", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-152907139", note = "© 2004 American Chemical Society.", revision_no = "11", abstract = "A series of novel Cu_2N_2 diamond core structures supported by S_2N- and P_2N- ligands\nhave been prepared. These diamond core systems exhibit a reversible\n1-electron redox process between a reduced Cu^1Cu^1 and a class Ill delocalized\nCu^(1.5)Cu^(1.5) state. Solid-state molecular structures have been obtained of both\nthe oxidized and reduced forms of this system. Minimal overall structural\nreorganization is observed between the reduced and oxidized states, though a\nmarked CuCu compression (-0.13 Å) does occur upon oxidation. As a result,\nunusually facile electron self-exchange arises. Magnetic, electrochemical and\nspectroscopic data, as well as their reactivity, will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47531, title ="Spin state control on four-coordinate cobalt(II)", author = "Jenkins, David M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 508", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-141502343", note = "© 2004 American Chemical Society.", revision_no = "12", abstract = "The spin state of transition metal complexes can control the kinetics of reactivity\nfor first row ions and has generated interest recently due to their potential\nmagnetic device applications. Because it is difficult to predict and alter the spin\nstate for first row transition metal complexes systematically, we set out to\nsynthesize a cobalt(ll) model system where low-spin, high-spin, and spincrossover\ncomplexes are readily accessible through exchange of a single\nmonodentate ligand. The tripodal ligand [PhBP_3] ([PhBP_3]=[PhB(CH_2PPh_2)_3]-)\nallows for simple pseudo-tetrahedral complexes to be prepared from\n[PhBP_3]CoX (X=I, Cl) via metathesis of aryl oxides, arylthiol oxides, or siloxides.\nThese complexes have been characterized both as solids and in solution by\nutilizing NMR, EPR, SQUID and X-ray diffraction. The magnetic and structural\nproperties of these complexes will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47521, title ="Synthesis and reactivity of nickel complexes supported by the tris(phosphino) borate ligand [PhB(Ch_2PPh_2)_3]^-", author = "MacBeth, Cora E. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 907", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-100808567", note = "© 2004 American Chemical Society.", revision_no = "11", abstract = "The chemistry of nickel complexes supported by phosphine donor ligands has\ngained recent interest due to the ability of these species to catalyze a variety of chemical transformations. A series of divalent, monovalent, and zerovalent nickel\ncomplexes supported by the strongly electron-releasing, monoanionic tris(phosphino)\nborate ligand, [PhBP_3]=[PhB(CH_2PPh 2)_3]^-, have been synthesized to\nexplore their coordination chemistry and reactivity. The characterization of these\ncomplexes will be presented, with comparisons drawn to isostructural nickel\ncomplexes supported by neutral phosphine ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47528, title ="Synthetic control of excited states: New ancillary ligands for cyclometalated Ir(III) complexes", author = "Li, Jian and Djurovich, Peter I.", journal = "Abstracts of Papers of the American Chemical Society", volume = "227", pages = "INOR 385", month = "March", year = "2004", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-134929197", note = "© 2004 American Chemical Society.", revision_no = "14", abstract = "Due to the strong spin-orbit coupling of Ir atom, many cyclometalated Ir\ncomplexes are phosphorescent at room temperature, making these materials\nleading candidates for dopant emitters in OLEO applications. One method of\nmodifying emission energies of Ir phosphors is to change the nature of\ncyclometalating ligand. This presentation will describe a new method of tuning\nthe lowest excited state energy by employing high triplet energy ancillary ligands\nfor Ir phosphors. Using different ancillary ligands, i.e., a family of pyrazolyl\nligands and their analogues, and the same cyclometalating ligand, paratolylpyridine\n(tpy), it is possible to vary the emission color of Ir phosphors over\na wide range. Electrochemical analysis shows that different ancillary ligands\nmodify the emission energy by affecting the energy of HOMO while leaving the\nLUMO relatively unchanged. Meanwhile, the Ir complexes appear to have longer\nluminescence lifetime and slower radiative decay when their emission energies\nincrease above some level.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47431, title ="Amido-Bridged Cu_2N_2 Diamond Cores that Minimize Structural Reorganization and Facilitate Reversible Redox Behavior between a Cu^1Cu^1 and a Class III Delocalized Cu^(1.5)Cu^(1.5) Species", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "126", number = "9", pages = "2885-2893", month = "March", year = "2004", doi = "10.1021/ja037364m", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-132520202", note = "© 2004 American Chemical Society.\n\nReceived July 17, 2003.\n\nThis work was funded by BP and the DOE (PECASE). We acknowledge Larry Henling for crystallographic assistance and Dr. Angel J. Di Bilio and David M. Jenkins for assistance with the EPR and SQUID magnetization studies.", revision_no = "13", abstract = "A novel Cu_(2)N_(2) diamond core structure supported by an [SNS]^(-) ligand (1) ([SNS]^(-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu1Cu1 complex, {[SNS][Cu]}_(2) (2), and a class III delocalized Cu^(1.5)Cu^(1.5) state, [{[SNS][Cu]}_(2_][B(3,5-(CF_(3))_(2)C_(6)H_(3))_(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu···Cu bond distance nonetheless undergoes an appreciable compression (0.13 Å) upon oxidation, providing a Cu···Cu distance of 2.4724(4) Å in the mixed-valence state that is virtually identical to the Cu···Cu distance observed in the reduced form of the Cu_(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS]^(-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS]^(-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange ks between the Cu^(1)Cu^(1) and the Cu^(1.5)Cu^(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for ks can be bracketed with a conservative lower boundary of ≥107 M^(-1) s^(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of ks likely reflects the minimal structural reorganization that accompanies Cu^(1)Cu^(1) ↔ Cu^(1.5)Cu^(1.5) interchange.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47680, title ="Platinum-alkyl and hydride complexes supported by a tris(phosphino)borate ligand: structural and spectroscopic studies", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Polyhedron", volume = "23", number = "2-3", pages = "489-497", month = "January", year = "2004", doi = "10.1016/j.poly.2003.11.036", issn = "0277-5387", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-150704289", note = "© 2003 Elsevier Ltd.\n\nReceived 30 July 2003; Accepted 8 October 2003.\n\nLarry Henling is acknowledged for his assistance with\nX-ray crystallography. Joseph Duimestra is acknowledged\nfor assistance with the protocol for [PhBP3] [TBA]. J.C.T. is grateful to the NSF and to the Moore Foundation for graduate research fellowships. The authors acknowledge financial support from the NSF (CHE-0132216) and the DOE (PECASE).", revision_no = "10", abstract = "The synthesis and characterization of platinum-methyl and hydride complexes containing the ligand [PhBP_3] ([PhBP_3]=[PhB(CH_2PPh_2)_3]^−) are described. Metalation of [PhBP_3][TBA] (1[TBA], [TBA]=[nBu_4N]^+) with platinum methyl precursors provided the corresponding phosphine methyl platinum species [PhBP_3]PtMe_3 (2) and [[κ^2-PhBP_3]PtMe_2][TBA] (3). The reactivity of octahedral 2 and square planar 3 are discussed, and their X-ray diffraction structures are reported. Octahedral Pt(IV) 2 was found to be very thermally and chemically robust. In contrast, the square planar Pt(II) complex 3 demonstrated reactivity at both the platinum center and the unbound phosphine. The reaction of 3 with one equivalent of elemental sulfur provided the corresponding phosphine-sulfide [[κ^2-PhB(CH_2P(S)Ph_2)(CH_2PPh_2)_2]PtMe_2][TBA] (4). Similarly, the reaction of 3 with one equivalent of BH_3 • SMe_2 generated the borane-protected complex [[κ^2-PhB(CH_2P(BH_3)Ph_2)(CH_2PPh_2)_2]PtMe_2][TBA] (5). Complex 5 was significantly less reactive than 3 to a variety of substrates under similar conditions; however, the reaction of 5 with additional BH_3 • SMe_2 resulted in the formation of the platinum(I) bridged hydride species [[κ^2-PhB(CH_2P(BH_3)Ph_2)(CH_2PPh_2)_2]Pt(μ-H)]_2[TBA]_2 (6).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47392, title ="Coordinating Anions: (Phosphino)tetraphenylborate Ligands as New Reagents for Synthesis", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "43", number = "1", pages = "8-10", month = "January", year = "2004", doi = "10.1021/ic0350234", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-104615116", note = "© 2004 American Chemical Society.\n\nReceived August 31, 2003.\n\nThe NSF (CHE-01232216) and BP have provided financial support for this work. Larry Henling and J. Christopher Thomas provided assistance with the crystallographic studies.", revision_no = "16", abstract = "Anionic, electron-releasing phosphines that incorporate a borate counteranion within the ligand framework are promising reagents for organometallic catalysis. This report describes the synthesis of a new class of monodentate tertiary phosphines built upon the commonly employed tetraphenylborate anion. These new phosphines\nare highly stable and strongly electron-releasing and readily coordinate transition metals. Moreover, they are promising reagents for catalysis, as demonstrated by their ability to promote the Suzuki cross-coupling of aryl chloride substrates.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47430, title ="Solution and Solid-State Spin-Crossover Behavior in a Pseudotetrahedral d^7 Ion", author = "Jenkins, David M. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "125", number = "37", pages = "11162-11163", month = "September", year = "2003", doi = "10.1021/ja036198f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-132440365", note = "© 2003 American Chemical Society.\n\nReceived May 16, 2003.\nPublication Date (Web): August 22, 2003.\n\nWe thank the NSF (CHE-0132216) and the\nDOE (PECASE) for financial support. D.M.J. acknowledges an NSF\ngraduate research fellowship. We are grateful to the Beckman\nInstitute (Caltech) for use of the SQUID magnetometer, and to Mike\nDay and Larry Henling for crystallographic assistance.", revision_no = "20", abstract = "This Communication describes a pseudotetrahedral d^(7) complex, [PhBP_(3)]Co(OSiPh_(3)), that exhibits thermally induced spin-crossover both in solution and in the solid state. Magnetic crossover behavior is achieved by confluence of the X-type ligand and the tripodal auxiliary employed. Four-coordinate platforms of the present geometry type may offer a new approach to magnetic spin-crossover behavior distinct from their electronically related pseudooctahedral counterparts.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47433, title ="Dinitrogen Chemistry from Trigonally Coordinated Iron and Cobalt Platforms", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "125", number = "36", pages = "10782-10783", month = "September", year = "2003", doi = "10.1021/ja036687f", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-132701006", note = "© 2003 American Chemical Society.\n\nReceived June 14, 2003.\n\nThis work was supported by a DOE PECASE award. T.A.B. is grateful for a Department of Defense graduate research fellowship.", revision_no = "14", abstract = "This report establishes that trigonally coordinated “[PhBP^(iPr)_(3)]M” platforms (M = Fe, Co) will support both π-acidic (N_2) and π-basic (NR) ligands at a fourth binding site. The N_2 complexes of iron that are described are the first thoroughly characterized examples to exhibit a 4-coordinate geometry. Methylation of monomeric {Fe^(0)(N_(2))-} and {Co^(0)(N_(2))-} species successfully derivatizes the β-N atom of the coordinated N_(2) ligand and affords the diazenido products {Fe^(II)(N_(2)Me)} and {CoII(N2Me)}, respectively. One-electron oxidation of the mononuclear M^(0)(N_(2))- species produces dinuclear and synthetically versatile M^(I)(N_(2))M^(I) complexes. These latter species provide clean access to the chemistry of the “[PhBP^(iPr)_(3)]M^(I)” subunit. For example, addition of RN_(3) to M^(I)(N_(2))M^(I) results in oxidative nitrene transfer to generate [PhBP^(iPr)_(3)]M_(≡)NR with concomitant N_(2) release. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47396, title ="Bis(phosphino)borates:\u2009 A New Family of Monoanionic Chelating Phosphine Ligands", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "42", number = "17", pages = "5055-5073", month = "August", year = "2003", doi = "10.1021/ic034150x", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-105001149", note = "© 2003 American Chemical Society.\n\nReceived February 12, 2003.\n\nThe authors acknowledge Dr. Biaxin Qian and Connie Lu for synthetic assistance. Larry Henling, Dr. Michael Day, and Theodore Betley are acknowledged for their assistance with X-ray crystallography. J.C.T. is grateful to the NSF and to the Moore Foundation for graduate research fellowships. The authors acknowledge financial support from the NSF (Grant CHE-0132216), the ACS Petroleum Research Fund, and the Dreyfus Foundation.", revision_no = "15", abstract = "The reaction of dimethyldiaryltin reagents Me_(2)SnR_(2) (R ) Ph (1), p-MePh (2), m,m-Me_(2)Ph (3), p-tBuPh (4), p-MeOPh\n(5), p-CF_(3)Ph (6)) with BCl_(3) provided a high-yielding, simple preparative route to the corresponding diarylchloroboranes R_(2)BCl (R ) Ph (10), p-MePh (11), m,m-Me2Ph (12), p-tBuPh (13), p-MeOPh (14), p-CF_(3)Ph (15)). In some cases, the desired diarylchloroborane was not formed from an appropriate tin reagent Me_(2)SnR_(2) (R ) o-MeOPh (7), o,o-(MeO)_(2)Ph (8), o-CF_(3)Ph (9)). The reaction of lithiated methyldiaryl- or methyldialkylphosphines with diarylchloroboranes or dialkylchloroboranes is discussed. Specifically, several new monoanionic bis(phosphino)borates\nare detailed: [Ph_(2)B(CH_(2)PPh_(2))_(2)] (25); [(p-MePh)_(2)B(CH_(2)PPh_(2))_(2)] (26); [(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)] (27); [(p-MeOPh)_(2)B-\n(CH_(2)PPh_(2))_(2)] (28); [(p-CF_(3)Ph)_(2)B(CH_(2)PPh_(2))_(2)] (29); [Cy_(2)B(CH_(2)PPh_(2))_(2)] (30); [Ph_(2)B(CH_(2)P{p-^(t)BuPh}_(2))_(2)] (31); [(p-MeOPh)_(2)B- (CH_(2)P{p-^(t)BuPh}_(2))_(2)] (32); [Ph_(2)B(CH_(2)P{p-CF_(3)Ph}_(2))_(2)] (33); [Ph_(2)B(CH_(2)P(BH_(3))(Me)_(2))_(2)] (34); [Ph_(2)B(CH_(2)P(S)(Me)_(2))_(2)] (35); [Ph_(2)B(CH_(2)PiPr_(2))_(2)] (36); [Ph_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (37); [(m,m-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (38). The chelation of diarylphosphine derivatives 25-33 and 36 to platinum was examined by generation of a series of platinum dimethyl complexes. The electronic effects of substituted bis(phosphino)borates on the carbonyl stretching frequency of neutral platinum alkyl carbonyl complexes were studied by infrared spectroscopy. Substituents remote from the metal center (i.e. on boron) have minimal effect on the electronic nature of the metal center, whereas substitution close to the metal center (on phosphorus) has a greater effect on the electronic nature of the metal center.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47582, title ="The Strong-Field Tripodal Phosphine Donor, [PhB(CH_2P^iPr_2)_3]^-, Provides Access to Electronically and Coordinatively Unsaturated Transition Metal Complexes", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "42", number = "17", pages = "5074-5084", month = "August", year = "2003", doi = "10.1021/ic0343096", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140730-084117504", note = "© 2014 American Chemical Society.\n\nReceived March 21, 2003.\n\nWe acknowledge the Dreyfus Foundation and the ACS Petroleum Research Fund for financial support. T.A.B. thanks the Department of Defense for a graduate research fellowship. J.C.P. is grateful for a Camille-Dreyfus Teacher Scholar Award.", revision_no = "10", abstract = "This paper introduces a sterically encumbered, strong-field tris(diisopropylphosphino)borate ligand, [PhBPiPr3] ([PhBP^(iPr)_(3)] = [PhB(CH_2P^iPr_2)_3]^-, to probe aspects of its conformational and electronic characteristics within a host of complexes. To this end, the Tl(I) complex, [PhBP^iPr_3]Tl (1), was synthesized and characterized in the solid-state by X-ray diffraction analysis. This precursor proves to be an effective transmetallating agent, as evidenced by its reaction with the divalent halides FeCl_2 and CoX_2 (X = Cl, I) to produce the monomeric, 4-coordinate, high-spin derivatives [PhBP^iPr_3]FeCl_2 and [PhBP^iPr_3]CoX (X = Cl_3, I_4) in good yield. Complexes 2−4 were each characterized by X-ray diffraction analysis and shown to be monomeric in the solid-state. For conformational and electronic comparison within a system exhibiting higher than 4-coordination, the 16-electron ruthenium complexes {[PhBP^iPr_3]Ru(μ-Cl)}_2 (5) and {[PhBP_3]Ru(μ-Cl)}_2 (6) were prepared and characterized ([PhBP_3] = [PhB(CH_2PPh_2)_3]^-. The chloride complexes 2 and 3 reacted with excess CO to afford the divalent, monocarbonyl adducts [PhBP^iPr_3]FeCl(CO) (7) and [PhBP^iPr_3]CoCl(CO) (8), respectively. Reaction of 4 with excess CO resulted in the monovalent, dicarbonyl product [PhBP^iPr_3]CoI(CO)_2 (9). Complexes 5 and 6 also bound CO readily, providing the octahedral, 18-electron complexes [PhBP^iPr_3]RuCl(CO)_2 (10) and [PhBP_3]RuCl(CO)_2 (11), respectively. Dimers 5 and 6 were broken up by reaction with trimethylphosphine to produce the mono-PMe_3 adducts [PhBP^iPr_3]RuCl(PMe_3) (12) and [PhBP3]RuCl(PMe3) (13). Stoichiometric oxidation of 3 with dioxygen provided the 4-electron oxidation product [PhB(CH_2P(O)^iPr_2)_2(CH_2P^iPr_2)]CoCl (14), while exposure of 3 to excess oxygen results in the 6-electron oxidation product [PhB(CH_2P(O)^iPr_2)_3]CoCl (15). Complexes 2 and 4 were characterized via cyclic voltammetry to compare their redox behavior to their [PhBP_3] analogues. Complex 4 was also studied by SQUID magnetization and EPR spectroscopy to confirm its high-spin assignment, providing an interesting contrast to its previously described low-spin relative, [PhBP_3]CoI. The difference in spin states observed for these two systems reflects the conformational rigidity of the [PhBPiPr3] ligand by comparison to [PhBP_3], leaving the former less able to accommodate a JT-distorted electronic ground state.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47432, title ="Zwitterionic and Cationic Bis(phosphine) Platinum(II) Complexes:\u2009 Structural, Electronic, and Mechanistic Comparisons Relevant to Ligand Exchange and Benzene C−H Activation Processes", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "125", number = "29", pages = "8870-8888", month = "July", year = "2003", doi = "10.1021/ja0296071", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-132627539", note = "© 2003 American Chemical Society.\n\nReceived December 5, 2002.\n\nThe authors acknowledge financial support from the NSF (CHE-0132216), BP, and the ACS PRF. J.C.T. is grateful for an NSF graduate fellowship and a Moore fellowship. Dr. Michael Day, Lawrence Henling, and Theodore Betley are acknowledged for assistance with crystallographic studies. Prof. John Bercaw, Dr. Alan Heyduk, Dr. Gregory Kubas, and Dr. Jennifer Love are acknowledged for insightful discussions.\nNote Added after ASAP: The version published on the Web\n06/17/2003 was not the corrected version. The version published on the Web 06/25/2003 and the print version are correct.", revision_no = "29", abstract = "Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph_(2)B(CH_(2)PPh_(2))_(2)], ([Ph_(2)BP_(2)], [1]), Ph_(2)Si(CH_(2)PPh_(2))_(2), (Ph_(2)SiP_(2), 2), and H_(2)C(CH_(2)PPh_(2))_(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph_(2)BP_(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph_(2)SiP_(2))Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, η3-binding mode of the [Ph_(2)BP_(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (15), each undergo a C−H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph_(2)BP_(2)]Pt(Ph)(THF) (16), [(Ph_(2)SiP_(2))Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (18). Examination of the kinetics of each C−H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C6H6)/k(C6D6):\u2009 13 = 1.26; 14 = 6.52; 15 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [{(Ph_(2)SiP_(2))Pt}2(μ-η3:η3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(μ-η^(3):η^(3)-biphenyl)][B(C_(6)F_(5))_(4)]_(2) (20). Extended thermolysis of neutral [Ph_(2)BP_(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt {[Ph_(2)BP_(2)]PtPh_(2)}-{[Ph_(2)BP_(2)]Pt(THF)_(2)}+. The bulky phosphine adducts [Ph_(2)BP_(2)]Pt(Me){P(C_(6)F_(5))_(3)} (25) and [(Ph_(2)SiP_(2))Pt(Me){P(C_(6)F_(5))_(3)}][B(C_(6)F_(5))_(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13−15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d6 shows only CH_(4) and CH3D. Whereas CH_(3)D is the dominant byproduct for 14, 15, 25, and 29, CH_(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative [Ph_(2)BP_(2)]Pt(^(13)CH_(3))(THF) (13-^(13)CH_(3)), and its deuterium-labeled derivative [Ph_(2)B(CH_(2)P(C_(6)D5)_(2))_(2)]Pt(Me)(THF) (13-d20), establish that reversible [Ph_(2)BP_(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d_(20)-labeled 13-d_(20) in benzene-d_(6) affords k_(13)/k_(13-d20) ~ 3. The NMR data obtained for 13, 13-^(13)CH_3, and 13-d_20 suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph_(2)BP_(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47552, title ="Zwitterionic Relatives to the Classic [(P–P)-Rh(solv)_2]^+ Ions: Neutral Catalysts Active for H—E Bond Additions to Olefins (E=C, Si, B)", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Angewandte Chemie International Edition", volume = "42", number = "21", pages = "2385-2389", month = "May", year = "2003", doi = "10.1002/anie.200250378", issn = "1433-7851", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140729-100252822", note = "© 2003 Wiley-VCH Verlag GmbH & Co. Article first published online: 28 May 2003. Manuscript Revised: 10 Feb 2003.\nManuscript Received: 16 Oct 2002. We thank the NSF (CHE-0132216), the ACS Petroleum Research Fund, and the Dreyfus Foundation for financial support. T.A.B. thanks the Department of Defense for a graduate research fellowship. Brian Stoltz and Jeremy May provided helpful discussions and a sample of 4-methyl-4-pentenal.", revision_no = "14", abstract = "Formally zwitterionic bis(phosphanyl)- and bis(amino)borate rhodium(I) complexes (see picture) can catalytically mediate the hydrogenation, hydroacylation, hydroboration, and hydrosilation of double bonds. These neutral systems are shown to be highly active, even under conditions incompatible with their isostructural, but formally cationic, relatives.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47522, title ="Dinitrogen chemistry of Ru and Os (quinolinyl)amido complexes", author = "Peters, Jonas C. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "225", pages = "INOR 891", month = "March", year = "2003", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140728-101201599", note = "© 2003 American Chemical Society.", revision_no = "22", abstract = "The well-developed chemistry of Ru and Os complexes supported by neutral\nN-donor ligands (e.g., see studies by H. Taube and by T. J. Meyer) has exposed\na host of chemically fascinating, multi-electron redox transformations. These\ninclude nitride coupling to form N_2 , azide attack at coordinated NO to generate\nN_2O and N_2 as byproducts, and oxidative O-atom transfer. To derive similarly\nrobust systems that are more reducing in nature, our group has developed a\nversatile family of electron-rich Ru and Os systems supported by anionic triand\nbidentate (quinolinyl)amido ligands. Our design scheme allows for selective\nsite exposure at a position trans to an amido functionality, chosen to stabilize a\nrange of oxidation states that should promote desirable group transfer activity.\nThese complexes do appear particularly well-suited to small molecule binding\n(e.g., N_2, O_2, N_2O). For example, mono- and dinuclear nitrogen complexes of Ru\nand Os can be systematically prepared under appropriate reaction conditions. In\nthis talk I plan to discuss the emerging chemistry of these Ru and Os systems\nby emphasizing their synthetic development and their interesting electronic\nproperties. A particular focus will be placed on these data with respect to\nevaluating the potential of these amido-based systems for N_2 reduction/\nfunctionalization chemistry.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52543, title ="Phosphino and amino borates: Stronger field donors for coordination chemistry", author = "Peters, Jonas C. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "225", pages = "INOR-435", month = "March", year = "2003", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-095726862", note = "© 2003 American Chemical Society.", revision_no = "10", abstract = "Our group has developed useful protocols to (phosphino)borate ligands\nfeaturing synthetic flexibility at both the borate and the phosphine donor\npositions. Expanding on these methods, we have also developed routes to\n(amino)borates. Representative examples of some ligands that we have prepared\nthus far are shown in the figure below. In this talk I plan to highlight interesting\nstructural, electronic, and reactivity properties of complexes supported by these\ntypes of ligands, each of which is characterized by a borate anion that is\npartially insulated from the coordinated metal center. For example, an understanding\nof subtle structural and electronic properties of a class of tris(phosphino)\nborate cobalt(II) complexes enables us to assign pseudo-tetrahedral\ncobalt(II) complexes that are either high or low spin, in addition to examples\nthat display temperature dependent spin-crossover. From a reactivity standpoint,\nwe have uncovered unusual group transfer processes that lead to molecular iron\nand cobalt imides, each of which undergoes subsequent nitrene release to CO\nas an acceptor substrate. In the case of iron in particular, the Fe(I)/Fe(III)\nnitrene group transfer cycle to CO (liberating isocyanate) can be made catalytic\nand both Fe(I) and Fe(III) species can be identified as intermediates in the cycle.\nOur continued efforts to explore these systems, and to expand this chemistry to\nother first row metals including nickel and copper, will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47957, title ="A Low-Spin d^5 Iron Imide: Nitrene Capture by Low-Coordinate Iron(I) Provides the 4-Coordinate Fe(III) Complex [PhB(CH_2PPh_2)_3]FetN-p-tolyl", author = "Brown, Steven D. and Betley, Theodore A.", journal = "Journal of the American Chemical Society", volume = "125", number = "2", pages = "322-323", month = "January", year = "2003", doi = "10.1021/ja028448i", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-081320174", note = "© 2003 American Chemical Society. \nPublication Date (Web): December 13, 2002. We thank the ACS PRF and the Dreyfus Foundation for financial support, and David M. Jenkins, Dr. Michael W. Day, and Dr. Angel J. Di Bilio for assistance.", revision_no = "16", abstract = "Entry into “[PhBP_3]Fe” chemistry affords a rare, pseudotetrahedral iron(I) complex, [PhBP_3]Fe(PPh_3), with an S = 3/2 ground state. This precursor undergoes rapid oxidation by aryl azide to produce the d^5 imide [PhBP_3]Fe_≡NAr (Ar = p-tolyl). The Fe(III) imide is significant in that it is low-spin and represents the first mononuclear imide of iron. Doublet [PhBP_3]Fe_≡NAr reacts rapidly and quantitatively with CO at room temperature to release isocyanate and [PhBP_3]Fe(CO)_2. The [PhBP_3]Fe(CO)_2 byproduct is also a precursor to [PhBP_3]Fe_≡NAr upon addition of aryl azide.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47416, title ="Elucidation of a Low Spin Cobalt(II) System in a Distorted Tetrahedral Geometry", author = "Jenkins, David M. and Di Bilio, Angel J.", journal = "Journal of the American Chemical Society", volume = "124", number = "51", pages = "15336-15350", month = "December", year = "2002", doi = "10.1021/ja026433e ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-090945029", note = "© 2002 American Chemical Society.\n\nReceived April 5, 2002. Revised Manuscript Received September 27, 2002.\n\nWe are grateful to the Dreyfus Foundation,\nthe ACS Petroleum Research Fund, and the National Science\nFoundation (Grant No. CHE-0132216) for financial support of\nthis work. We thank the Beckman Institute (Caltech) for use of\nthe SQUID magnetometer and the crystallographic facility.\nD.M.J. is grateful for a pre-doctoral fellowship from the National\nScience Foundation. M.J.A. is grateful for a NDSEG Fellowship.\nT.A.B. is grateful for a pre-doctoral fellowship from the DOD.\nJ. C. Thomas is acknowledged for assistance with DFT. Finally,\nwe acknowledge our inorganic colleagues at Caltech for many\nstimulating discussions, and the reviewers of this paper for\nhelpful commentary.", revision_no = "16", abstract = "We have prepared a series of divalent cobalt(II) complexes supported by the [PhBP_3] ligand ([PhBP_3] = [PhB(CH_2PPh_2)_3]-) to probe certain structural and electronic phenomena that arise from this strong field, anionic tris(phosphine) donor ligand. The solid-state structure of the complex [PhBP_3]CoI (1), accompanied by SQUID, EPR, and optical data, indicates that it is a pseudotetrahedral cobalt(II) species with a doublet ground state the first of its type. To our knowledge, all previous examples of 4-coordinate cobalt(II) complexes with doublet ground states have adopted square planar structure types. Complex 1 provided a useful precursor to the corresponding bromide and chloride complexes, {[PhBP_3]Co(μ-Br)}_2, (2), and {[PhBP_3]Co(μ-Cl)}_2, (3). These complexes were similarly characterized and shown to be dimeric in the solid-state. In solution, however, the monomeric low spin form of 2 and 3 dominates at 25 °C. There is spectroscopic evidence for a temperature-dependent monomer/dimer equilibrium in solution for complex 3. Furthermore, the dimers 2 and 3 did not display appreciable antiferromagnetic coupling that is typical of halide and oxo-bridged copper(II) and cobalt(II) dimers. Rather, the EPR and SQUID data for solid samples of 2 and 3 suggest that they have triplet ground states. Complexes 1, 2, and 3 are extremely oxygen sensitive. Thus, stoichiometric oxidation of 1 by dioxygen produced the 4-coordinate, high spin complex [PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]CoI, (4), in which the [PhBP_3] ligand had undergone a 4-electron oxidation. Reaction of 1 with TlOAr (Ar = 2,6-Me_2Ph) afforded an example of a 4-coordinate, high spin complex, [PhBP_3]Co(O-2,6-Me_2Ph) (5), with an intact [PhBP_3] ligand. The latter two complexes were spectroscopically and structurally characterized for comparison to complexes 1, 2, and 3. Our data for these complexes collectively suggest that the [PhBP_3] ligand provides an unusually strong ligand-field to these divalent cobalt complexes that is chemically distinct from typical tris(phosphine) donor ligand sets, and distinct from tridentate borato ligands that have been previously studied. Coupling this strong ligand-field with a pronounced axial distortion away from tetrahedral symmetry, a geometric consequence that is enforced by the [PhBP_3] ligand, provides access to monomeric [PhBP_3]CoX complexes with doublet rather than quartet ground states.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47964, title ="Synthesis of the (Dialkylamino)borate, [Ph_2B(CH_2NMe_2)_2]-, Affords Access to N-Chelated Rhodium(I) Zwitterions", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Inorganic Chemistry", volume = "41", number = "25", pages = "6541-6543", month = "November", year = "2002", doi = "10.1021/ic0259336", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140805-095114062", note = "© 2002 American Chemical Society. Received August 6, 2002. Publication Date (Web): November 12, 2002. We thank the NSF (Grant CHE-0132216), the ACS Petroleum Research Fund, and the\nDreyfus Foundation for financial support. T.A.B. thanks the\nDepartment of Defense for a graduate research fellowship.", revision_no = "13", abstract = "This paper reports the synthesis of the first bis(amino)borate ligand, [Ph_2B(CH_2NMe_2)_2]-, an anionic equivalent of tertiary diamines. Anionic [Ph_2B(CH_2NMe_2)_2] is an excellent bidentate ligand auxiliary and is used to prepare a series of N-chelated, zwitterionic rhodium(I) complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/79906, title ="Activations of Silanes with [PhB(CH_2PPh_2)_3]Ir(H)(η^3-C_8H_(13)). Formation of Iridium Silylene Complexes via the Extrusion of Silylenes from Secondary Silanes R_2SiH_2", author = "Feldman, Jay D. and Peters, Jonas C.", journal = "Organometallics", volume = "21", number = "20", pages = "4065-4075", month = "September", year = "2002", doi = "10.1021/om020389u", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170808-080407504", note = "© 2002 American Chemical Society. \n\nReceived 13 May 2002. Published online 31 August 2002. Published in print 1 September 2002. \n\nAcknowledgment is made to the National Science Foundation for their generous support of this work. We than Dr. Fred Hollander of the UC Berkeley CHEXRAY facility for assistance with the X-ray structure determinations and for solving the structures for 4a and 8b.", revision_no = "16", abstract = "The abstraction of methide from (Me_3P)_3Ir(SiHMes_2)(Me)(H) (1) with B(C_6F_5)_3 gave the silylene complex [fac-(Me_3P)_3(H)_2Ir(SiMes_2)][MeB(C_6F_5)_3] (2) via 1,2-hydrogen migration. Secondary silanes (H_2SiR_2) reacted with [PhBP_3]Ir(H)(η^3-C_8H^(13)) (3) (where [PhBP_3] = PhB(CH_2PPh_2)_3^-) to give silylene complexes of the type [PhBP_3](H)_2Ir═SiR_2 (R = 2,4,6-trimethylphenyl (Mes), 4a; R = Ph, 4b; R = Et, 4c; R = Me, 4d), with loss of cyclooctene. Analogously, the germylene complex [PhBP_3](H)2Ir═GeMes_2 was obtained via the reaction of 3 with Mes_2GeH_2. Primary silanes (H_3SiR) reacted with 3 to give [PhBP_3](H)_2Ir═Si(R)(c-C_8H_(15)) (R = Mes, 8a; R = 2,4,6-triisopropylphenyl (Trip), 8b) via an intermediate silylene complex with an Si−H bond, [PhBP_3](H)_2Ir═Si(R)(H). With tertiary silanes (R_3SiH), silyl-capped trihydride complexes of the type [PhBP_3]IrH_3(SiR_3) (R = Et, 7a; R = Me, 7b) and 1,3-cyclooctadiene were produced. The mechanisms of these processes are discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/74974, title ="Structural and Chemical Properties of Zwitterionic Iridium Complexes Featuring the Tripodal Phosphine Ligand [PhB(CH_2PPh_2)_3]^-", author = "Feldman, Jay D. and Peters, Jonas C.", journal = "Organometallics", volume = "21", number = "20", pages = "4050-4064", month = "September", year = "2002", doi = "10.1021/om0205086", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20170309-101305346", note = "© 2002 American Chemical Society. \n\nReceived 26 June 2002. Published online 31 August 2002. Published in print 1 September 2002. \n\nAcknowledgment is made to the National Science Foundation for their generous support of this work. We thank Dr. Fred Hollander of the UC Berkeley CHEXRAY facility for assistance with the X-ray structure determinations and for solving the structure of 2.", revision_no = "15", abstract = "Several new iridium compounds bearing the PhB(CH_2PPh_2)_3^- (herein abbreviated as [PhBP_3]) ligand have been prepared and characterized, and a comparison of steric, electronic, and chemical properties is made with those of related pentamethylcyclopentadienyl (Cp^*) and hydridotris(3,5-dimethylpyrazolyl)borate (Tp^(Me)_2) complexes. The complexes [PhBP_3]Ir(H)(η_3-C_8H_(13)) (2) and [PhBP_3]Ir(H)(η^3-C_3H_5) (3) were synthesized from the reaction of [Li(TMED)][PhBP_3] (1) with the corresponding [(alkene)_2IrCl]_2 complex. These allyl complexes serve as precursors to the dihalides [PhBP_3]IrX_2 (10, X = I; 12, X = Cl). In addition to these dihalides, the five-coordinate species [PhBP_3]IrMe_2 (16) and [ClB(CH_2PPh_2)_3]IrCl_2 (13) have been isolated. Addition of CO to 2 or 3 gave [PhBP_3]Ir(CO)_2 (7), while reaction of H_2 with 2 yielded {[PhBP_3]IrH_2}_2 (8) in benzene and [PhBP_3]Ir(COE)H_2 (9) in THF (where COE = cyclooctene). Complex 2 reacted with PMePh_2 to give [PhBP_3]Ir(PMePh_2)H_2 (5) and 1,3-cyclooctadiene. The protonation of 5 with [H(OEt_2)]{B[3,5-C_6H_3(CF_3)_2]_4} gave the classical hydride complex {[PhBP_3]Ir(PMePh_2)H_3}{B[3,5-C_6H_3(CF_3)_2]_4} (6). In addition to the formation of allyl complexes 2 and 3, several C−H activation reactions have been observed; addition of P^(Me)_3 to 2 provided the cyclometalated product {PhB[(CH_2PPh_2)_2(CH_2PPhC_6H_4)]}Ir(H)(PMe_3)(4) and COE. Photolysis of 5 gave PhB[(CH_2PPh_2)_2(CH_2PPhC_6H_4)]}Ir(H)(PMePh_2) (A) and [PhBP_3]Ir(H)(PMePhC_6H_4) (B). Complex 9 catalyzes H/D exchange between COE and benzene-d_6. Metathesis reactions of diiodide 10 with LiBHEt_3 gave [Li(THF)_n]{[PhBP_3]Ir(H)_2I} (14a) and [Li(THF)_n]{[PhBP_3]Ir(H)_3} (15). Comparison of the spectroscopic properties of related [PhBP_3]Ir, Cp^*Ir, and Tp^(Me)_2Ir complexes suggests that relative donating abilities follow the trend [PhBP_3] ≥ Cp^* > Tp^(Me)_2, and structural comparisons indicate that [PhBP_3] is the most sterically demanding ligand.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47419, title ="Oxidative Group Transfer to Co(I) Affords a Terminal Co(III) Imido Complex", author = "Jenkins, David M. and Betley, Theodore A.", journal = "Journal of the American Chemical Society", volume = "124", number = "38", pages = "11238-11239", month = "September", year = "2002", doi = "10.1021/ja026852b", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-094451859", note = "© 2002 American Chemical Society.\n\nReceived May 9, 2002.\n\nWe thank the NSF (CHE-0132216), the ACS PRF, and the Dreyfus Foundation for financial support. D.M.J. is grateful for an NSF predoctoral fellowship. The authors acknowledge J. Christopher Thomas for assistance with preliminary DFT\ncalculations and Dr. Daniel Mindiola for insightful discussions.", revision_no = "13", abstract = "The synthesis of the first terminal imido complex of cobalt, [PhBP3]CoN-p-tolyl, is reported. Its synthesis proceeds by oxidative group transfer from cobalt(I) upon addition of tolyl azide at room temperature. This species and a related η1-diazoalkane adduct have been structurally characterized. The diamagnetic imido complex [PhBP3]CoN-p-tolyl reacts with CO to liberate isocyanate and the cobalt(I) dicarbonyl complex [PhBP_(3)]Co(CO)_(2). ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47499, title ="(Quinolinyl)amido ligands for late transition metal chemistry", author = "Peters, Jonas C. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 21", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140725-130445793", note = "© 2002 American Chemical Society.", revision_no = "11", abstract = "By contrast to earlier metals, the chemistry of late transition metal amido\ncomplexes are poorly developed. Our group is exploring the chemistry of a\nseries of bi- and tridentate (quinolinyl)amido ligands that support robust group\n8, 9, and 10 metal complexes in a range of oxidation states. This talk will\ndiscuss the coordination chemistry of these systems, and will highlight their\nutility for novel transformations.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47502, title ="Development of chiral anionic bis(phosphino)borate ligands for catalytic asymmetric synthesis", author = "Collier, Philip N. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 320", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140725-131521540", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "Cationic, coordinatively unsaturated late metal centres occupy a critical role in\nthe development of asymmetric catalysis. Our group has recently prepared a\nfamily of formally zwiterionic group 9 and 10 complexes supported by anionic\nbis(phosphino)borate ligands to study neutral species that afford chemistry\nsimilar to more conventional cationic systems. We now hope to extend this\nchemistry to asymmetric catalysis using chiral bis(phosphino)borate ligands (of\ngeneral structure 1). The synthetic challenges to be met and our recent progress\non this front will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47475, title ="Examining [R2B(pz)2]Pt(X)(L) complexes for sigma bond activation", author = "Thomas, Christine M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 565", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140724-141343200", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "The chemistry of platinum supported by tris(pyrazolyl)borate complexes has\nbecome an area of intense interest with respect to sigma bond activation\nchemistry (for example, see recent work by the respective groups of Goldberg,\nTempleton, and Brookhart). In contrast, surprisingly little bis(pyrazolyl)borate\nGroup 10 chemistry has been explored. Divalent platinum complexes for this\nligand class are synthetically challenging in their own right: we have begun to\ntarget their systematic preparation. Our goal is to compare the reactivity of\n[R2B(pz)2]Pt(X)(L) complexes with bis(phosphino)borate complexes of the type\n[Ph2BP2]Pt(X)(L), systems that are also active with respect to sigma bond\nactivation chemistry. Recent progress with respect to the preparation and study\nof [R2B(pz)2]Pt(X)(L) complexes will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52544, title ="Selective activation of Sp^3 C-H bonds of tertiary amines", author = "Lu, Connie C. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR-561", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-100631418", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "Our group is interested in C-H bond activation by late transition metals supported\nby the anionic bis(phosphino)borate ligand, Ph_2B(CH_2PPh_2)_2 (abbreviated\nPh_2BP_2). In this talk, we focus on the reactivity of the complex\n[(Ph_2BP_2)Pd(THF)_2][0Tf] (1). Compound 1 reacts with tertiary amines via\nC-H bond activation to generate a three-membered Pd-N-C metallacycle. The\nscope of this reaction will be discussed, as well as strategies for futher\nsubstrate functionalization.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47501, title ="Small molecule coordination chemistry by ruthenium and osmium complexes of bis(quinolinyl)amido ligands", author = "Betley, Theodore A. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 633", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140725-131123916", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "The monoanionic, tridentate bis(8-quinolinyl)amide (BOA) ligand has been\nemployed as a ligand scaffold for divalent ruthenium and osmium complexes.\nExposing a coordination site on the metal provides access to coordination of\nsmall molecules, with dinitrogen and dioxygen substrates targeted in particular.\nSmall molecule complexation and group transfer processes for the \"(BQA)M\"\ncomplexes have been explored, the results of which will be presented (M=Ru, Os).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47500, title ="Synthesis of cobalt complexes supported by tris(phosphino)borate", author = "Jenkins, David M. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 631", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140725-130843808", note = "© 2002 American Chemical Society.", revision_no = "11", abstract = "The tris(phosphino)borate ligand, [PhBP_3]-, is unusual in that it is a very strong\nfield donor ligand that can support unsaturated and electron deficient transition\nmetal complexes. For example, it can be used to support distorted tetrahedral\ncobalt(ll) complexes that are low spin, distinct from the many high spin systems\npreviously reported. Our attempts to study the reactivity of \"[PhBP_3]Co\" systems to\nexplore group transfer and small molecule activation chemistry will be highlighted.\nTwo illustrative examples of reactivity readily afforded by the novel low\nspin [PhBP_3]Col complex are shown below. Stoichiometric oxygen addition\naffords a novel high spin cobalt iodide in which the donor ligand has undergone\na formal 4-electron oxidation. The low spin iodide complex also binds CO\nreadily, chemically distinct from the many 4-coordinate high spin cobalt(ll)\nsystems known.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47470, title ="Zwitterionic complexes supported by an anionic bis(phosphino)borate ligand: Bond activation studies and novel transformations", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "224", pages = "INOR 556", month = "August", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140724-131732244", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "The anionic, bidentate phosphine ligand [Ph_2B(CH_2PPh_2)_2], ([Ph_2BP_2]), and\nfunctionalized derivatives of these bis(phosphino)borates have been employed as\nauxiliary ligands in the preparation of complexes of platinum and copper, whose\nreaction chemistry has been examined. For example, the neutral complex\n(Ph_2BP_2)Pt(Me)(THF) gives rise to several examples of E-H bond activation\nchemistry (E=C, Si, H, B). Moreover, copper derivatives supported by functionalized\nbis(phosphino)borate ligands appear to be promising candidates for group\ntransfer reactions. Our efforts to elucidate the bond activation and/or atom and\ngroup transfer processes of these zwitterionic complexes will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47393, title ="Catalytic Copolymerization of CO and Ethylene with a Charge Neutral Palladium(II) Zwitterion", author = "Lu, Connie C. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "124", number = "19", pages = "5272-5273", month = "May", year = "2002", doi = "10.1021/ja017011s", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-104719734", note = "© 2002 American Chemical Society.\n\nReceived September 6, 2001.\n\nWe thank the NSF (CHE-0132216) and the Dreyfus Foundation for funding. Dr. Michael Day and Lawrence Henling are acknowledged for crystallographic assistance, Mona Shahgholi for MALDI-TOF, and Rapra Technology Limited for GPC analyses of polyketone.", revision_no = "13", abstract = "The synthesis of a zwitterionic Pd(II) complex supported by an anionic bis(phosphino)borate ligand, Ph_(2)B(CH_(2)PPh_(2))_(2) (abbreviated as [Ph_(2)BP_(2)]), is reported. The new complex, [Ph_(2)BP_(2)]PdMe(THF), is active for CO and ethylene copolymerization. The copolymerization activity and polyketone molecular weight for the neutral, zwitterionic system are compared with those for the cationic systems [R2E(CH2PPh2)2PdMe(THF)][B(C_(6)F_(5))_(4)] where ER_(2) = SiPh_(2) and CH_(2). Surprisingly, the more electron rich zwitterionic system is a catalyst of activity comparable to that of the more conventional cationic systems.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47399, title ="Base-Promoted Benzene C−H Activation Chemistry at an Amido Pincer Complex of Platinum(II)", author = "Harkins, Seth B. and Peters, Jonas C.", journal = "Organometallics", volume = "21", number = "9", pages = "1753-1755", month = "April", year = "2002", doi = "10.1021/om011044z", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-122514070", note = "© 2002 American Chemical Society.\n\nReceived December 7, 2001; Publication on Web 04/02/2002. \n\nWe thank BP, Caltech, and the Dreyfus Foundation for financial support of this work and Larry Henling for crystallographic assistance. Members of the Bercaw group are thanked for insightful\ndiscussions.", revision_no = "12", abstract = "The thermally robust platinum(II) complex (BQA)Pt(OTf) undergoes benzene C−H bond activation at 150 °C but requires the presence of N^iPr_2Et. The reaction products are the phenyl complex (BQA)Pt(Ph) and a stoichiometric equivalent of [HN^iPr_2Et][OTf].", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47474, title ="(Phosphino)borate coordination chemistry", author = "Peters, Jonas C. and Jenkins, David M.", journal = "Abstracts of Papers of the American Chemical Society", volume = "223", pages = "INOR 234", month = "April", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140724-134232603", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "We are studying a host of complexes that are supported by (phosphino)borate\nligands. For example, coordination of a tris(phosphino)borate ligand to a\ndivalent cobalt(ll) center gives rise to a low spin, cobalt(ll) species that is\n4-coordinate, psuedo-tetrahedral, and monomeric. Virtually all related complexes\nare high spin. Moreover, this low spin species can be directly converted to a\n4-coordinate, high spin relative by clean oxidation of the phosphine donor\nligand. Aspects of this and/or related work will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47473, title ="New strategies for C-H activation chemistry at late metal centers", author = "Peters, Jonas C. and Harkins, Seth B.", journal = "Abstracts of Papers of the American Chemical Society", volume = "223", pages = "INOR 162", month = "April", year = "2002", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140724-133714120", note = "© 2002 American Chemical Society.", revision_no = "10", abstract = "To help elucidate factors that are important to alkane activation, our group is\ndeveloping new inorganic systems that engage and activate aryl and alkyl C-H\nbonds. We have recently developed several late metal systems that participate in\nintra- and intermolecular C-H bond activation chemistry. These systems are\nsupported by auxiliary ligand sets including (phosphino)borates, (amino)borates,\nand pincer-like amides. This presentation will focus on the synthesis and\nreaction profile for complexes derived from these ligand families. Moreover, an\nattempt will be made to experimentally correlate complex reactivity/selectivity\nwith (i) the transition metal used, (ii) the complex's charge, and (iii) the\nauxiliary donor ligand employed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/27880, title ="A homoleptic phosphine adduct of Tl(I)", author = "Shapiro, Ian R. and Jenkins, David M.", journal = "Chemical Communications", volume = "2001", number = "20", pages = "2152-2153", month = "September", year = "2001", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20111121-084449767", note = "© 2001 The Royal Society of Chemistry. Received (in Cambridge, UK) 21st May 2001, Accepted 22nd August 2001.\nFirst published as an Advance Article on the web 27th September 2001. We thank the Dreyfus Foundation, Caltech, and the NSF for financial support of this work.", revision_no = "25", abstract = "A homoleptic phosphine adduct of thallium(I) supported by a tris(phosphino)borate ligand has been isolated and structurally characterized.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47697, title ="Pincer-like Amido Complexes of Platinum, Palladium, and Nickel", author = "Peters, Jonas C. and Harkins, Seth B.", journal = "Inorganic Chemistry", volume = "40", number = "20", pages = "5083-5091", month = "September", year = "2001", doi = "10.1021/ic010336p", issn = "0020-1669", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140731-091158681", note = "© 2001 American Chemical Society. Publication Date (Web): August 24, 2001. Received March 27, 2001. We wish to thank the California Institute\nof Technology and the Dreyfus Foundation for their generous\nsupport of this research through a start-up grant and a Dreyfus\nFoundation New Faculty Award.", revision_no = "15", abstract = "The ligands bis(8-quinolinyl)amine (BQAH, 1), (2-pyridin-2-yl-ethyl)-(8-quinolinyl)amine (2-pyridin-2-yl-ethyl-QAH, 2), o-dimethylaminophenyl(8-quinolinyl)amine (o-(NMe_2)Ph-QAH, 3), and 3,5-dimethylphenyl(8-quinolinyl)amine (3,5-Me_2Ph-QAH, 4) have been prepared in high yield from aryl halide and amine precursors by palladium-catalyzed coupling reactions. Deprotonation of 1 with ^nBuLi in toluene affords the lithium amide complex [Li][BQA] (5), whose dimeric solid-state crystal structure is presented. Lithium amide 5 was transmetalated by TlOTf to afford the thallium(I) amido complex [Tl][BQA] (6). An X-ray structural study of 6 shows it to be a 1:1 complex of the BQA ligand and Tl. Entry into the group 10 chemistry of the parent ligand 1 was effected by both protolytic and metathetical strategies. Thus, the divalent chloride complexes (BQA)PtCl (7), (BQA)PdCl (8), and (BQA)NiCl (9) were prepared and fully characterized. An X-ray structural study for each of these three complexes shows them to be well-defined, square-planar complexes in which the auxiliary BQA ligand binds in a planar, ^η3-fashion. For comparison, the reactivity of ligands 2−4 with (COD)PtCl_2 was studied. While reaction with ligand 2 afforded an ill-defined product mixture, ligands 3 and 4 reacted with (COD)PtCl_2 to generate the unusual alkyl complexes (o-(NMe_2)Ph-QA)Pt(1,2-η^2-6-σ-cycloocta-1,4-dienyl) (10) and (3,5-Me_2Ph-QA)Pt(1,2-η^2-6-σ-cycloocta-1,4-dienyl) (11), both of which have been structurally characterized. ", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47442, title ="(Phosphino)borate chemistry of the late transition elements", author = "Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "222", pages = "INOR 309", month = "August", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-154221099", note = "© 2001 American Chemical Society.", revision_no = "10", abstract = "Our research group has prepared a series of late metal complexes supported by\n(phosphino)borate ligands. This presentation will discuss the preparative organometallic chemistry of these compounds, and will emphasize the preparation\nand reactivity of charge neutral, zwitterionic species supported by (phosphino)\nborate ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47441, title ="Anionic amido and borato ligands for transition metal chemistry", author = "Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "222", pages = "INOR 257", month = "August", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-153844590", note = "© 2001 American Chemical Society.", revision_no = "11", abstract = "Our group is exploring transition metal chemistry supported by multidentate\namido and borato ligands. These ligands will be discussed from the perspective\nof their synthesis and the metal-based reaction chemistry that they support.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47444, title ="Late metal zwitterions for catalytic and stoichiometric transformations", author = "Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "222", pages = "CATL 28", month = "August", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-154455889", note = "© 2001 American Chemical Society.", revision_no = "10", abstract = "Our research group is targeting reactive, charge neutral zwitterions of the late\ntransition elements; we hope to expose reaction chemistry reminiscent al\nelectrophilic or truly cationic late metal centers. Recently we prepared an\nanionic, bidentate phosphine ligand and studied its ability to support C-H\nactivation chemistry at a divalent platinum center. Hoping to explore new\nsystems relevant to polymerization catalysis and organic transformations we are\nnow targeting related palladium and nickel species. For example, we have\nprepared a palladium(ll) alkyl complex. {Ph_2B(CH_2PPh_2)_2}Pd(Me)(THF), strategically designed to mimic the highly active, cationic palladium copolymerization\ncatalyst [(dppp)PdMe(OEt_2)][Bar_4]. This presentation will address the following\ngeneral question: Can charge neutral complexes containing borate counter-anions\ntethered to but insulated from late metal centers be used for desirable\nstoichiometric and catalytic transformations. Furthermore, how does the\nchemistry of these neutral zwitterions compare with that of their more familiar\ndiscrete salt or ion-paired counterparts?", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52534, title ="Atomic Carbon as a Terminal Ligand: Studies of a Carbidomolybdenum Anion Featuring Solid-State ^(13)C NMR Data and Proton-Transfer Self-Exchange Kinetics", author = "Greco, Jane B. and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "123", number = "21", pages = "5003-5013", month = "May", year = "2001", doi = "10.1021/ja003548e", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-093751439", note = "© 2001 American Chemical Society.\n\nReceived September 29, 2000.\n\nThe authors are grateful for support from the National Science Foundation (CHE-9988806) and for fellowships to C.C.C. from the Alfred P. Sloan and the David and Lucile Packard Foundations. C.C.C. also wishes to thank the National Science Board for awarding him the 1998 Alan T.\nWaterman Award. J.B.G. is grateful for a predoctoral fellowship from the National Science Foundation, J.C.P. appreciated greatly a predoctoral fellowship from the Department of Defense, and T.A.B. benefited from the support of the MIT UROP office. Also deserving of thanks are Dr. Daniel Mindiola for help with X-ray crystallography, Dr. Jeff Simpson for helpful discussions\nregarding solution NMR spectroscopy, and Professor Klaus\nTheopold for suggesting the possibility that proton transfer was occurring on the NMR time scale. We are grateful to Professor Joseph Templeton for providing information prior to publication. G. W. wishes to thank the Natural Sciences Engineering Research Council (NSERC) of Canada for research and equipment grants. We also are grateful to Professor Almeria Natansohn (Queen’s University) for providing access to the ASX200 spectrometer.", revision_no = "14", abstract = "Anion [CMo(N[R]Ar)_3]- (R = C(CD3)_2CH_3 or tBu, Ar = 3,5-C_6H_3Me_2) containing one-coordinate carbon as a terminal substituent and related molecules have been studied by single-crystal X-ray crystallography, solution and solid-state ^(13)C NMR spectroscopy, and density functional theory (DFT) calculations. Chemical reactivity patterns for [CMo(N[R]Ar)_3]- have been investigated, including the kinetics of proton-transfer self-exchange involving HCMo(N[R]Ar)_3, the carbidomolybdenum anion's conjugate acid. While the Mo⋮C bond lengths in [K(benzo-15-crown-5)_2][CMo(N[R]Ar)_3] and the parent methylidyne, HCMo(N[R]Ar)_3, are statistically identical, the carbide chemical shift of δ 501 ppm is much larger than the δ 282 ppm shift for the methylidyne. Solid-state ^(13)C NMR studies show the carbide to have a much larger chemical shift anisotropy (CSA, 806 ppm) and smaller 95Mo−13C coupling constant (60 Hz) than the methylidyne (CSA = 447 ppm, 1J_MoC = 130 Hz). DFT calculations on model compounds indicate also that there is an increasing MoC overlap population on going from the methylidyne to the terminal carbide. The pKa of methylidyne HCMo(N[R]Ar)_3 is approximately 30 in THF solution. Methylidyne HCMo(N[R]Ar)3 and carbide [CMo(N[R]Ar)3]- undergo extremely rapid proton-transfer self-exchange reactions in THF, with k = 7 × 10^6 M^(-1) s^(-1). Besides being a strong reducing agent, carbide [CMo(N[R]Ar)_3]- reacts as a nucleophile with elemental chalcogens to form carbon−chalcogen bonds and likewise reacts with PCl_3 to furnish a carbon−phosphorus bond.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47387, title ="Benzene C−H Activation at a Charge Neutral Zwitterionic Platinum(II) Complex", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Journal of the American Chemical Society", volume = "123", number = "21", pages = "5100-5101", month = "May", year = "2001", doi = "10.1021/ja0058987 ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-093726094", note = "© 2001 American Chemical Society.\n\nReceived December 18, 2000.\nPublication Date (Web): May 5, 2001.\n\nWe thank the California Institute of Technology\nfor its generous support of this research through a start-up grant. J.C.P.\nthanks the Dreyfus Foundation for a Dreyfus New Faculty Award. J.C.T.\nthanks the National Science Foundation and the California Institute of\nTechnology for financial support. The authors thank Prof. John Bercaw\nfor helpful discussions and Dr. Michael Day and Lawrence Henling for\ntheir assistance with X-ray crystallography.", revision_no = "11", abstract = "Cationic, coordinatively unsaturated metal centers exhibit a\nwide range of both stoichiometric and catalytic transformations.\nSuch species are frequently generated by methide abstraction with\na strong Lewis acid, or alternatively by protonation with an acid\nwhose conjugate base is noncoordinating or weakly coordinating.3\nWe are targeting charge neutral, zwitterionic complexes that\ndisplay reaction chemistry reminiscent of these reactive metal\ncenters. Of particular interest is the development of species that\nexhibit transformations at X-H bonds, where an X-H bond refers\ngenerally to a C-H or other robust σ bond. A conceptual diagram\nillustrating this strategy is shown in Scheme 1.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47407, title ="Bond Activation Studies at Zwitterionic Platinum(II) Alkyl Complexes Supported by an Anionic Bisphosphinoborate Ligand", author = "Thomas, J. Christopher and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 82", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-073248803", note = "© 2001 American Chemical Society.", revision_no = "9", abstract = "The anionic, bidentate phosphine ligand [Ph_2B(CH_2PPh_2))_2]. ([Ph_2BP_2]), has been\nprepared and employed as an auxiliary ligand for species of the type\n[(Ph_2BP_2)PtXY] (X, Y=alkyl, aryl, halide). C-H and other bond activation\nprocesses for (Ph_2BP_2)PtYL complexes have been elucidated, the results of\nwhich will be presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47413, title ="Synthesis and reactivity of late transition metal complexes of bis(8-quinolinyl)amine", author = "Harkins, Seth B. and Brown, Steven D.", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 94", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-081237120", note = "© 2001 American Chemical Society.", revision_no = "11", abstract = "A markedly improved synthesis for the previously reported bis(8-quinolinyl)amine\n(BQA) ligand has been developed utilizing Pd-coupling methods recently\ndeveloped by the groups of Buchwald and Hartwig. This synthesis allows ready\naccess to multigram quantities of bis(8-quinolinyl)amine, and the development\nof a host of Ni, Pd, & Pt chemistry supported by the monoanionic, tridentate\namido ligand will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47439, title ="Synthesis and reactivity of late transition metal complexes supported by quinoline based ligands", author = "Brown, Steven D. and Harkins, Seth B.", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 202", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-153121881", note = "© 2001 American Chemical Society.", revision_no = "9", abstract = "Buchwald and Hartwig Pd-coupling methods have been utilized for the multi-gram\nsyntheses of quinoline-based bi- and tridentate monoanionic ligands. The\nformation of complexes using suitable late metal precursors and these ligands\nwill be presented from a structural perspective. Additionally, the initial reaction\nchemistry of these complexes will be discussed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47428, title ="Synthesis and reactivity of novel ruthenium complexes supported by chelating amido ligands", author = "Betley, Theodore A. and Thomas, J. Christopher", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 198", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-125517531", note = "© 2001 American Chemical Society.", revision_no = "11", abstract = "A series of BQA (BQA=Bis(8-quinolinyl)amide) supported ruthenium complexes\nhave been prepared with a range of accompanying auxiliary ligands, including\nphosphines, amides, and alkoxides. Their structures and chemical reactivity with\na series of small molecule substrates will be addressed.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47411, title ="Synthesis and reactivity of ruthenium bis(8-quinolinyl)amide complexes", author = "Qian, Baixin and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 239", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-075236228", note = "© 2001 American Chemical Society.", revision_no = "10", abstract = "The coordination chemistry of ruthenium supported by bis(8-quinolinyl)amide\n(BOA) ligand was investigated. Treatment of Ru(COD)Cl_2 (COD=1,5-cyclooctadiene)\nwith one equivalent of lithium bis(8-quinolinyl)amide in toluene afforded\nRu(BQA)(COD)CI. A range of reaction chemistry will be reported for\nRu(BQA)(COD)CI and related complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47412, title ="Transition metal complexes supported by (8-quinolynyl)amido ligands", author = "Peters, Jonas C. and Betley, Theodore A.", journal = "Abstracts of Papers of the American Chemical Society", volume = "221", pages = "INOR 662", month = "April", year = "2001", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140723-080337837", note = "© 2001 American Chemical Society.", revision_no = "12", abstract = "Our program is targeting the preparation of reactive transition metal complexes\nsuitable for the activation and functionalization of small molecule substrates. As part of this effort we have prepared a series of anionic, chelating amido ligands\nderived from 8-aminoquinoline utilizing Pd-coupling methods. Thus far, a host\nof Group VIII, IX, and X transition metal systems has been developed from the\nparent BQA (BQA = bis(8-quinolynyl)amide) ligand, such as those examples\ndepicted below. The preparative chemistry enabling access to these and several\nsecond generation systems will be discussed, in addition to their unfolding\nreaction chemistry.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52545, title ="Transition metal complexes supported by tris(phosphino) borate ligands", author = "Peters, Jonas C. and Allen, Matthew", journal = "Abstracts of Papers of the American Chemical Society", volume = "220", pages = "INOR-539", month = "August", year = "2000", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-101201170", note = "© 2000 American Chemical Society.", revision_no = "9", abstract = "Our group is currently targeting charge neutral metal complexes that exhibit\nchemistry reminiscent of cationic, coordinatively unsaturated species. Within this\ncontext we have set out to prepare new metal complexes bearing anionic\ntris-phosphino borate ligands, [PhB(CH2PPh2)3]-, and have thus far succeeded\nin isolating and characterizing a range of molecular complexes ranging from the\nmid to latter part of the transition series. For example, the cobalt(II) complexes\n{PhB(CH2PPh2)3]CoX}2 (X=CI, Br) have been prepared and shown to be dimeric\nin the solid state. The related iodide complex PhB(CH2PPh2)3]Col, however, is\nrigorously monomeric in the solid state and represents an unusual 15 electron\ncobalt complex. The physical and chemical properties of these and other\ncompounds will be discussed, in addition to our recent efforts at expanding the\ntoolkit of tris-phosphino borate ligands.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47389, title ="Synthesis and Structure of Li[(C_5H_4)CH_2CH_2(TACN-^iPr_2)]. A Lithium Complex Supported by a Cp/TACN-^iPr_2 Ligand", author = "Qian, Baixin and Henling, Lawrence M.", journal = "Organometallics", volume = "19", number = "14", pages = "2805-2808", month = "July", year = "2000", doi = "10.1021/om000147z", issn = "0276-7333", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-095258009", note = "© 2000 American Chemical Society.\n\nReceived February 15, 2000; Publication on Web 05/31/2000.\n\nWe wish to thank the California Institute of Technology for its generous support of this research through a start-up grant. We would like to express our thanks to Professor John Arnold for discussing related results with us prior to publication. Finally, Mr. Matthew Allen is acknowledged for his assistance with the preparation of TACN-^iPr_2.", revision_no = "15", abstract = "The new ligand (C_5H_5)CH_2CH_2(TACN-^iPr_2) (TACN-^iPr_2 = 4,7-diisopropyl-1,4,7-triaza-1-cyclononyl) has been prepared and structurally characterized as its lithium complex, Li[(C_5H_4)CH_2CH_2(TACN-^iPr_2)]. The method of preparation employed nucleophilic addition of the lithium amide complex [Li(TACN-^iPr_2)]_2 to spiro[2.4]hepta-4,6-diene. The crystal structure of [Li(TACN-^iPr_2)]_2 is also presented.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/47400, title ="Silylene extrusion from silanes: Direct conversion of R2SiH2 to iridium silylene dihydride complexes", author = "Feldman, Jay D. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "219", pages = "INOR 368", month = "March", year = "2000", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20140722-130800070", note = "© 2000 American Chemical Society.", revision_no = "13", abstract = "Silylene complexes, the silicon analogues of carbene complexes, feature\nthree-coordinate, divalent silicon bound to a metal (R_2Si=ML_n). Significant\nadvances in the preparation of these highly electrophilic species have been\nmade in our laboritories over the past decade including, most recently, the\npreparation of the first iridium silylene complex as a zwitterion:\n[PhB(CH_2PPh_2)_3]lr(H)(eta-3-C_8H_(13)) + Mes_2SiH_2-[PhB(CH_2PPh_2)_3](H)_2lr=SiMes_2.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52551, title ="Dimolybdenum–µ-cyanide complexes supported by N-tert-butylanilide ligation: in pursuit of cyanide reductive cleavage", author = "Peters, Jonas C. and Baraldo, Luis M.", journal = "Journal of Organometallic Chemistry", volume = "59", number = "1-2", pages = "24-35", month = "December", year = "1999", doi = "10.1016/S0022-328X(99)00485-4", issn = "0022-328X", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-115043556", note = "© 1999 Elsevier Science S.A. Received 15 June 1999; accepted 6 August 1999.", revision_no = "12", abstract = "The red cyanide complex (NC)Mo(N[R]Ar)_3 (R=C(CD_3)_2CH_3, Ar=3,5-C_6H_3Me_2) was prepared in 77% yield by reaction of iodide IMo(N[R]Ar)_3 with tetra-n-butyl ammonium cyanide. By reaction of cyanide (NC)Mo(N[R]Ar)_3 with the three-coordinate molybdenum(III) complex Mo(N[R]Ar)_3 was prepared the dimolybdenum-μ-cyanide complex (μ-CN){Mo(N[R]Ar)_3}_2 as a violet solid in 80% yield. Reduction of μ-cyanide (μ-CN){Mo(N[R]Ar)_3}_2 by one electron would give a cyanide-bridged anion isoelectronic with the known μ-N_2 complex (μ-N_2){Mo(N[R]Ar)_3}_2, an intermediate in dinitrogen cleavage to two equivalents of nitride NMo(N[R]Ar)_3 by Mo(N[R]Ar)_3. Instead of undergoing an analogous cleavage of cyanide upon one-electron reduction, μ-cyanide (μ-CN){Mo(N[R]Ar)_3}_2 was found to undergo expulsion of a ligand C(CD_3)_2CH_3 substituent upon exposure to reducing conditions, the product isolated in 50% yield being imido–μ-cyanide (Ar[R]N)_2(ArN)Mo(μ-NC)Mo(N[R]Ar)_3. By reaction of [N^nBu_4][CN] with the 1-adamantyl-substituted molybdenum complex Mo(N[Ad]Ar)_3, the blue salt [N^nBu_4][(NC)Mo(N[Ad]Ar)_3] was obtained in 91% yield. Reaction of ferrocenium triflate or silver triflate with [N^nBu_4][(NC)Mo(N[Ad]Ar)_3] gave ferrocene or silver along with the neutral cyanide complex (NC)Mo(N[Ad]Ar)_3, isolated in 74% yield. While reaction of (NC)Mo(N[Ad]Ar)_3 with Mo(N[R]Ar)_3 gave in 53% yield a burgundy-colored dimolybdenum–μ-cyanide complex (Ar[Ad]N)_3Mo(μ-CN)Mo(N[R]Ar)_3. the 1-adamantyl-substituted cyanide did not exhibit any reaction with the 1-adamantyl-substituted tricoordinate complex Mo(N[Ad]Ar)_3. The latter results indicate that cyanide is too small to serve as a bridge for two equivalents of the highly sterically encumbered Mo(N[Ad]Ar)_3 fragment. A metathetical route to a heterodinuclear cyanide-bridged complex was explored involving addition of [N^nBu_4][(NC)Mo(N[Ad]Ar)_3] to the vanadium iodide complex IV(N[R]Ar_F)_2. By this reaction was obtained the orange–brown μ-cyanide complex (Ar_F[R]N)_2V(μ-NC)Mo(N[Ad]Ar)_3 in 30% recrystallized yield. The latter was characterized by X-ray crystallography. The cyanide chemistry reported here is interpreted with the aid of bonding considerations and cyclic voltammetry studies on the new complexes.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52537, title ="Redox-Catalyzed Binding of Dinitrogen by Molybdenum N-tert-Hydrocarbylanilide Complexes: Implications for Dinitrogen Functionalization and Reductive Cleavage", author = "Peters, Jonas C. and Cherry, John-Paul F.", journal = "Journal of the American Chemical Society", volume = "121", number = "43", pages = "10053-10067", month = "November", year = "1999", doi = "10.1021/ja991435t", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-094058286", note = "© 1999 American Chemical Society.\n\nReceived May 3, 1999.\n\nFor support of this work, the authors are grateful to the National Science Foundation (CAREER Award CHE-9501992), the Packard Foundation, and the Alfred P. Sloan Foundation. Additional funding for which the authors are profoundly grateful has come from the National Science Board (Alan T. Waterman Award to C.C.C., 1998).", revision_no = "15", abstract = "The splitting of dinitrogen (1 atm, THF, 25 °C) by Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3, Ar = 3,5-C_6H_3Me_2) giving 2 equiv of nitride N⋮Mo(N[R]Ar)3 is found to be accelerated in the presence of sodium amalgam. Careful control of the Mo(N[R]Ar)_3 concentration led to the isolation and characterization of the anionic dinitrogen complex, [(THF)xNa][(N_2)Mo(N[R]Ar)_3], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(THF)xNa][(N2)Mo(N[R]Ar)_3] is found to be a key intermediate in the acceleration of N_2 splitting by Mo(N[R]Ar)_3 in the presence of sodium amalgam. Accordingly, in the presence of an electron acceptor, [(THF)xNa][(N_2)Mo(N[R]Ar)_3] reacts with Mo(N[R]Ar)_3 to give the neutral N2-bridged complex (μ-N_2){Mo(N[R]Ar)_3}_2, which in turn splits to 2 equiv of nitride N⋮Mo(N[R]Ar)3. It is seen that the function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar)_3 to (μ-N2){Mo(N[R]Ar)3}2, a dinuclear dinitrogen complex that does not lose N_2 readily. Electrochemical or chemical outer-sphere oxidation of [(THF)xNa][(N2)Mo(N[R]Ar)_3] leads to rapid N_2 evolution with regeneration of Mo(N[R]Ar)_3, presumably via the neutral mononuclear dinitrogen complex (N2)Mo(N[R]Ar)_3. In situ generated [(THF)xNa][(N_2)Mo(N[R]Ar)_3] was efficiently trapped by ClSiMe3 to give (Me3SiNN)Mo(N[R]Ar)_3. This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me_2NN)Mo(N[R]Ar)_3][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar)_3 also was achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N_2)Mo(N[R]Ar)_3 gave rise to efficient syntheses of heterodinuclear dinitrogen complexes including (Ph[tBu]N)3Ti(μ-N_2)Mo(N[R]Ar)_3, which also was synthesized in its ^(15)N_2-labeled form. Synthesis and characterization data for the new N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar)_3 (Ad = 1-adamantyl, Ar = 3,5-C_6H_3Me_2) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 °C) in the presence of sodium amalgam to give the dinitrogen anion complex [(THF)xNa][(N_2)Mo(N[Ad]Ar)_3]; the synthesis does not require careful regulation of the Mo(N[Ad]Ar)_3 concentration. Indeed, under no conditions has Mo(N[Ad]Ar)_3 been observed to split dinitrogen or to give rise to a dinuclear μ-N_2 complex; this striking contrast with the reactivity of Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52540, title ="Reductive trapping of dinitrogen by molybdenum tris anilide to form a dinitrogen anion and chemistries thereof", author = "Cherry, John-Paul F. and Peters, Jonas C.", journal = "Abstracts of Papers of the American Chemical Society", volume = "218", pages = "INOR-560", month = "August", year = "1999", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-095135539", note = "© 1999 American Chemical Society.", revision_no = "12", abstract = "The isolation and characterization of the anoinic dinitrogen complex, [Na][N_2Mo(N[t-Bu]Ar)_3] (Ar=3,5 dimethylaryl)\nhas been accomplished. The capture of dinitrogen also occurs during the formation of\n(RR'N)_3TiN_2Mo(N[t-Bu]Ar)_3 where R=t-Bu and R'=Ph or R=R'=Me. Activation of small molecules with\nthese dinitrogen complexes has been studied encompassing an array of CS_2 and CO_2 reactions. Elimination\nof dinitrogen from the anionic complex formed the η^2 complex, (Ar[t-Bu]N)_3Mo(CS_2SiMe_3).", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/28959, title ="A terminal molybdenum carbide prepared by methylidyne deprotonation", author = "Peters, Jonas C. and Odom, Aaron L.", journal = "Chemical Communications", volume = "1997", number = "20", pages = "1995-1996", month = "October", year = "1997", issn = "1359-7345", url = "https://resolver.caltech.edu/CaltechAUTHORS:20120125-111723380", note = "© 1997 Royal Society of Chemistry.\nReceived in Bloomington, IN, USA, 17th June 1997; 7/04251E.\n", revision_no = "14", abstract = "The carbide anion [CMo{N(R)Ar}_3]– [R = C(CD_3)_2CH_3, Ar = C_6H_3Me_2-3,5], is obtained by deprotonation of the corresponding methylidyne compound, [HCMo{N(R)Ar}_3], and is characterized by X-ray diffraction as its {K(benzo-15-crown-5)_2}+ salt, thereby providing precedent for the carbon atom as a terminal substituent in transition-metal chemistry.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52546, title ="Atom-abstraction as a synthetic tool", author = "Cummins, Christopher C. and Odom, Aaron L.", journal = "Abstracts of Papers of the American Chemical Society", volume = "214", pages = "INOR-136", month = "September", year = "1997", issn = "0065-7727", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141210-102137883", note = "© 1997 American Chemical Society.", revision_no = "9", abstract = "Three-coordinate complexes of formula M(N[R]Ar)_3 (R = C(CD_3)_2CH_3; Ar = 3,5-\nC_6H_3Me_2) have been found to serve as halogen (M = Ti) and nitrogen or phosphorus\n(M =Mo) abstraction agents. A variety of reactions have been investigated with the\nintent of delineating chemistry arising as a consequence of atom-abstraction. As an\nexample, reaction of Mo(N[R]Ar)_3 with N=Mo(S-1 adamantyl)_3 leads to intermetal\nN-atom transfer via an observable nitrido-bridged intermediate.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52407, title ="Assembly of Molybdenum/Titanium μ-Oxo Complexes via Radical Alkoxide C−O Cleavage", author = "Peters, Jonas C. and Johnson, Adam R.", journal = "Journal of the American Chemical Society", volume = "118", number = "42", pages = "10175-10188", month = "October", year = "1996", doi = "10.1021/ja960564w ", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141204-145154416", note = "© 1996 American Chemical Society.\n\nReceived February 21, 1996.\n\nPublication Date (Web): October 23, 1996.\n\nFor funding, C.C.C. thanks the National\nScience Foundation (CAREER Award CHE-9501992), DuPont\n(Young Professor Award), the Packard Foundation (Packard\nFoundation Fellowship), Union Carbide (Innovation Recognition Award), and 3M (Innovation Fund Award). J.C.P. is grateful\nfor a Department of Defense graduate research fellowship.", revision_no = "14", abstract = "Three-coordinate Ti(NRAr)_3 [R = C(CD_3)_2(CH_3), Ar = C_6H_3Me_2] was prepared in 73% yield by sodium amalgam reduction of ClTi(NRAr)_3 and in 83% yield upon treatment of TiCl_3(THF)_3 with 3 equiv of Li(NRAr)(OEt_2) in the presence of TMEDA. Ti(^tBuNPh)_3 was prepared similarly in 75% yield by treatment of TiCl_3(THF)_3 with 3 equiv of Li(^tBuNPh)(OEt_2) in the presence of TMEDA. Reaction of Ti(NRAr)_3 with NMo(O^tBu)_3 in hydrocarbon solvents at −35 °C generates a thermally unstable intermediate formulated as (^tBuO)_3Mo[μ-N]Ti(NRAr)_3, which readily loses a tert-butyl radical and isomerizes at 25 °C. Kinetics of the latter process were obtained over the temperature range 20−60 °C; the process exhibits clean first-order behavior. The following activation parameters were obtained:\u2009 ΔH⧧ = 21.4 ± 0.2 kcal mol^(-1) and ΔS⧧ = −3.7 ± 0.6 cal mol^(-1) K^(-1). The oxo-bridged product (^tBuO)_2(N)Mo[μ-O]Ti(NRAr)_3 was isolated in 83% yield from this reaction. Full characterization of the latter diamagnetic complex included an X-ray crystal structure and an ^(15)N NMR study. Ti(NRAr)_3 (1 equiv) reacts further with (^tBuO)_2(N)Mo[μ-O]Ti(NRAr)_3 to generate a species formulated as a second paramagnetic nitrido-bridged intermediate, (^tBuO)_2Mo{[μ-O]Ti(NRAr)_3}{[μ-N]Ti(NRAr)_3}, which at 25 °C loses a tert-butyl radical and isomerizes to give the final product, (^tBuO)(N)Mo{[μ-O]Ti(NRAr)_3}_2, isolated as an orange powder in 91% yield. Characterization of the latter diamagnetic complex included an ^(15)N NMR study. Attempts to displace a third tert-butyl radical by treatment of (^tBuO)(N)Mo{[μ-O]Ti(NRAr)_3}_2 with Ti(NRAr)_3 led to no reaction. Treatment of (^tBuO)(N)Mo{[μ-O]Ti(NRAr)_3}_2 with neat methyl iodide led to the isolation of (MeO)(N)Mo{[μ-O]Ti(NRAr)_3}_2 in 51% yield; ^(13)C and nitrido-^(15)N derivatives of this species were prepared for spectroscopic characterization. O_2Mo{[μ-O]Ti(^tBuNPh)_3}_2 was prepared in 59% yield upon treatment of MoO_2(O^tBu)_2 with 2 equiv of Ti(^tBuNPh)_3 in benzene at 65 °C. Full characterization of O_2Mo{[μ-O]Ti(^tBuNPh)_3}_2 included a single-crystal X-ray diffraction study. Previously reported (^iPrO)_3V[μ-O]Ti(NRAr)_3 was oxidized with ferrocenium triflate to give TfOTi(NRAr)_3 and OV(O^iPr)_3. TfOTi(NRAr)_3 was prepared independently in 80% yield by treatment of Ti(NRAr)_3 with ferrocenium triflate. (^iPrO)_3V[μ-O]Ti(NRAr)_3 is stable in the presence of methyl iodide. ITi(NRAr)_3 was prepared independently by treatment of Ti(NRAr)_3 with the stoichiometric amount of iodine. Paramagnetic (^tBuO)_3V[μ-O]Ti(NRAr)_3 was prepared as orange-brown needles in 94% yield and was found to be thermally stable. The relatively robust μ-nitrido compound (Me_2N)_3Mo[μ-N]Ti(^tBuNPh)_3, which was prepared in 77% isolated yield, showed no decomposition when heated in benzene at 70 °C for 13 h.", } @article {CaltechAUTHORS_https://authors.library.caltech.edu/id/eprint/52411, title ="Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes:\u2009 Mechanistic and Structural Data", author = "Laplaza, Catalina E. and Johnson, Marc J. A.", journal = "Journal of the American Chemical Society", volume = "118", number = "36", pages = "8623-8638", month = "September", year = "1996", doi = "10.1021/ja960574x", issn = "0002-7863", url = "https://resolver.caltech.edu/CaltechAUTHORS:20141204-152547970", note = "© 1996 American Chemical Society.\n\nReceived February 22, 1996.\nPublication Date (Web): September 11, 1996.\n\nThis work made use of MRSEC Shared\nFacilities supported by the National Science Foundation under\nAward No. DMR-9400334. The Stanford Synchrotron Radiation\nLaboratory is funded by the Department of Energy, Office\nof Basic Energy Sciences. The Biotechnology Program is\nsupported by the National Institutes of Health, Biomedical\nResearch Technology Program, Division of Research Resources.\nFurther support is provided by the Department of Energy, Office\nof Health and Environmental Research. We are indebted to\nRoger C. Prince of Exxon Research and Engineering Company\nfor his assistance in data collection and Martin J. George of\nSSRL for use of his data collection software. We are also\ngrateful to John Rehr of the University of Washington for\nproviding us with a special extended k-range version of his\nprogram feff. Raman spectra were collected in the Harrison\nSpectroscopy Laboratory at MIT, supported in part by the\nNational Institutes of Health. C.C.C. thanks Professors Richard\nR. Schrock and Barry K. Carpenter for helpful discussions. For\nfunding, C.C.C. thanks the National Science Foundation\n(CAREER Award CHE-9501992), DuPont (Young Professor\nAward), the Packard Foundation (Packard Foundation Fellowship),\nUnion Carbide (Innovation Recognition Award), and 3M\n(Innovation Fund Award). C.E.L. thanks MIT’s UROP (Undergraduate\nResearch Opportunities Program) for funding. M.J.A.J.\nis grateful for an NSERC graduate research fellowship.", revision_no = "12", abstract = "The synthesis and characterization of the complexes Mo[N(R)Ar]_3 (R = C(CD_3)_2CH_3, Ar = 3,5-C_6H_3Me_2), (μ-N_2){Mo[N(R)Ar]_3}_2, (μ-^(15)N_2){Mo[N(R)Ar]_3}_2, NMo[N(R)Ar]_3, ^(15)NMo[N(R)Ar]_3, Mo[N(t-Bu)Ph]_3, (μ-N_2){Mo[N(t-Bu)Ph]_3}_2, and NMo[N(t-Bu)Ph]_3 are described. Temperature-dependent magnetic susceptibility data indicate a quartet ground state for Mo[N(R)Ar]_3. Single-crystal X-ray diffraction studies for Mo[N(R)Ar]_3 and NMo[N(t-Bu)Ph]_3 are described. Extended X-ray absorption fine structure (EXAFS) structural studies for Mo[N(R)Ar]_3, (μ-N_2){Mo[N(R)Ar]_3}_2, and NMo[N(R)Ar]_3 are reported. Temperature-dependent kinetic data are given for the unimolecular fragmentation of (μ-N_2){Mo[N(R)Ar]_3}_2 to 2 equiv of NMo[N(R)Ar]_3 and for the fragmentation of (μ-^(15)N_2){Mo[N(R)Ar]_3}_2 to 2 equiv of ^(15)NMo[N(R)Ar]_3. The temperature dependence of the ^(15)N_2 isotope effect for the latter N_2 cleavage process was fitted to a simple harmonic model, leading to a prediction for the difference in NN stretching frequencies for the two isotopomers. The latter prediction was consistent with the Raman spectroscopic data for (μ-N_2){Mo[N(R)Ar]_3}_2 and (μ-^(15)N_2){Mo[N(R)Ar]_3}_2. The Raman spectroscopic data and EXAFS results are both consistent with an NN bond order of approximately 2 in (μ-N_2){Mo[N(R)Ar]_3}_2. Temperature-dependent magnetic susceptibility data consistent with a triplet ground state are given for (μ-N_2){Mo[N(t-Bu)Ph]_3}_2.", }